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Inverse Function Questions in English

Class 12 Mathematics · Relation and Function · Inverse Function

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Showing 49 of 144 questions in English

1
MediumMCQ
Let $A = \{1, 2, 3\}$ and $B = \{1, 3, 5\}$. $A$ relation $R: A \to B$ is defined by $R = \{(1, 3), (1, 5), (2, 1)\}$. Then ${R^{-1}}$ is defined by:
A
$\{(1, 2), (3, 1), (1, 3), (1, 5)\}$
B
$\{(1, 2), (3, 1), (2, 1)\}$
C
$\{(3, 1), (5, 1), (1, 2)\}$
D
None of these

Solution

(C) The inverse relation ${R^{-1}}$ is defined as the set of all ordered pairs $(y, x)$ such that $(x, y) \in R$.
Given $R = \{(1, 3), (1, 5), (2, 1)\}$.
By swapping the elements of each ordered pair in $R$,we get:
$(1, 3) \to (3, 1)$
$(1, 5) \to (5, 1)$
$(2, 1) \to (1, 2)$
Therefore,${R^{-1}} = \{(3, 1), (5, 1), (1, 2)\}$.
Thus,the correct option is $C$.
2
EasyMCQ
If $y = f(x) = \frac{ax + b}{cx - a}$,then $x$ is equal to
A
$1/f(x)$
B
$1/f(y)$
C
$yf(x)$
D
$f(y)$

Solution

(D) Given the function $y = f(x) = \frac{ax + b}{cx - a}$.
To find $x$ in terms of $y$,we perform the following steps:
$y(cx - a) = ax + b$
$cxy - ay = ax + b$
$cxy - ax = ay + b$
$x(cy - a) = ay + b$
$x = \frac{ay + b}{cy - a}$
Since $f(x) = \frac{ax + b}{cx - a}$,replacing $x$ with $y$ gives $f(y) = \frac{ay + b}{cy - a}$.
Therefore,$x = f(y)$.
3
MediumMCQ
Which of the following functions is invertible?
A
$f(x) = 2^x$
B
$f(x) = x^3 - x$
C
$f(x) = x^2$
D
None of these

Solution

(A) function $f: A \to B$ is invertible if and only if it is a bijection,meaning it is both one-one (injective) and onto (surjective).
$1$. For $f(x) = 2^x$,if we consider the domain as $\mathbb{R}$ and codomain as $(0, \infty)$,the function is strictly increasing (one-one) and covers all positive real values (onto). Thus,it is invertible.
$2$. For $f(x) = x^3 - x$,the derivative $f'(x) = 3x^2 - 1$ changes sign at $x = \pm \frac{1}{\sqrt{3}}$,so it is not monotonic and not one-one on $\mathbb{R}$.
$3$. For $f(x) = x^2$,$f(1) = f(-1) = 1$,so it is not one-one.
Therefore,$f(x) = 2^x$ is the correct answer.
4
EasyMCQ
If $y = f(x) = \frac{x + 2}{x - 1}$,then $x = $
A
$f(y)$
B
$2f(y)$
C
$\frac{1}{f(y)}$
D
None of these

Solution

(A) Given the function $y = \frac{x + 2}{x - 1}$.
To find $x$ in terms of $y$,we perform the following steps:
$y(x - 1) = x + 2$
$yx - y = x + 2$
$yx - x = y + 2$
$x(y - 1) = y + 2$
$x = \frac{y + 2}{y - 1}$
Since $f(y) = \frac{y + 2}{y - 1}$,we conclude that $x = f(y)$.
5
EasyMCQ
Which of the following functions is the inverse of itself?
A
$f(x) = \frac{1 - x}{1 + x}$
B
$f(x) = 5^{\log x}$
C
$f(x) = 2^{x(x - 1)}$
D
None of these

Solution

(A) function $f$ is the inverse of itself if $f(f(x)) = x$ for all $x$ in its domain.
For option $A$,let $f(x) = \frac{1 - x}{1 + x}$.
Then $f(f(x)) = f\left(\frac{1 - x}{1 + x}\right) = \frac{1 - \left(\frac{1 - x}{1 + x}\right)}{1 + \left(\frac{1 - x}{1 + x}\right)}$.
Multiplying the numerator and denominator by $(1 + x)$,we get:
$f(f(x)) = \frac{(1 + x) - (1 - x)}{(1 + x) + (1 - x)} = \frac{1 + x - 1 + x}{1 + x + 1 - x} = \frac{2x}{2} = x$.
Since $f(f(x)) = x$,the function $f(x) = \frac{1 - x}{1 + x}$ is the inverse of itself.
6
DifficultMCQ
The inverse of the function $f(x) = \frac{e^x - e^{-x}}{e^x + e^{-x}} + 2$ is given by
A
$\log_e \left( \frac{x - 2}{x - 1} \right)^{1/2}$
B
$\log_e \left( \frac{x - 1}{3 - x} \right)^{1/2}$
C
$\log_e \left( \frac{x}{2 - x} \right)^{1/2}$
D
$\log_e \left( \frac{x - 1}{x + 1} \right)^{-2}$

Solution

(B) Let $y = f(x) = \frac{e^x - e^{-x}}{e^x + e^{-x}} + 2$.
Multiply the numerator and denominator of the fraction by $e^x$:
$y = \frac{e^{2x} - 1}{e^{2x} + 1} + 2$.
Subtract $2$ from both sides:
$y - 2 = \frac{e^{2x} - 1}{e^{2x} + 1}$.
Let $u = e^{2x}$. Then $y - 2 = \frac{u - 1}{u + 1}$.
$(y - 2)(u + 1) = u - 1$
$yu + y - 2u - 2 = u - 1$
$u(y - 2 - 1) = -1 - y + 2$
$u(y - 3) = 1 - y$
$u = \frac{1 - y}{y - 3} = \frac{y - 1}{3 - y}$.
Since $u = e^{2x}$,we have $e^{2x} = \frac{y - 1}{3 - y}$.
Taking the natural logarithm on both sides:
$2x = \log_e \left( \frac{y - 1}{3 - y} \right)$
$x = \frac{1}{2} \log_e \left( \frac{y - 1}{3 - y} \right) = \log_e \left( \frac{y - 1}{3 - y} \right)^{1/2}$.
Thus,the inverse function is $f^{-1}(x) = \log_e \left( \frac{x - 1}{3 - x} \right)^{1/2}$.
7
DifficultMCQ
If the function $f:[1, \infty) \to [1, \infty)$ is defined by $f(x) = 2^{x(x - 1)}$,then $f^{-1}(x)$ is
A
$(\frac{1}{2})^{x(x - 1)}$
B
$\frac{1}{2}(1 + \sqrt{1 + 4\log_2 x})$
C
$\frac{1}{2}(1 - \sqrt{1 + 4\log_2 x})$
D
Not defined

Solution

(B) Given $f(x) = 2^{x(x - 1)}$.
To find the inverse,let $y = f(x) = 2^{x(x - 1)}$.
Taking $\log_2$ on both sides,we get $\log_2 y = x(x - 1)$.
This simplifies to the quadratic equation $x^2 - x - \log_2 y = 0$.
Using the quadratic formula $x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$,we get $x = \frac{1 \pm \sqrt{1 + 4\log_2 y}}{2}$.
Since the domain of $f$ is $[1, \infty)$,we must have $x \ge 1$.
If we choose the negative sign,$x = \frac{1 - \sqrt{1 + 4\log_2 y}}{2} < 1$ for $y \ge 1$.
Thus,we must choose the positive sign: $x = \frac{1 + \sqrt{1 + 4\log_2 y}}{2}$.
Replacing $y$ with $x$,we get $f^{-1}(x) = \frac{1}{2}(1 + \sqrt{1 + 4\log_2 x})$.
8
EasyMCQ
If $f(x) = 3x - 5$,then ${f^{ - 1}}(x)$ is:
A
Given by $\frac{1}{{3x - 5}}$
B
Given by $\frac{{x + 5}}{3}$
C
Does not exist because $f$ is not one-one
D
Does not exist because $f$ is not onto

Solution

(B) Let $f(x) = y$.
Since $f(x) = 3x - 5$,we have $y = 3x - 5$.
To find the inverse,we solve for $x$ in terms of $y$:
$y + 5 = 3x$
$x = \frac{y + 5}{3}$.
By definition,$x = f^{-1}(y)$,so $f^{-1}(y) = \frac{y + 5}{3}$.
Replacing $y$ with $x$,we get $f^{-1}(x) = \frac{x + 5}{3}$.
Since $f(x) = 3x - 5$ is a linear function,it is both one-one (injective) and onto (surjective),therefore $f$ is invertible.
9
EasyMCQ
If $f: \mathbb{R} \to \mathbb{R}$ is defined by $f(x) = 3x - 4$,then ${f^{ - 1}}: \mathbb{R} \to \mathbb{R}$ is
A
$4 - 3x$
B
$\frac{x + 4}{3}$
C
$\frac{1}{3x - 4}$
D
$\frac{3}{x + 4}$

Solution

(B) Given $f(x) = 3x - 4$.
To find the inverse function ${f^{ - 1}}(x)$,let $y = f(x)$.
Then $y = 3x - 4$.
Solving for $x$ in terms of $y$:
$y + 4 = 3x$
$x = \frac{y + 4}{3}$.
By definition,if $y = f(x)$,then $x = {f^{ - 1}}(y)$.
Therefore,${f^{ - 1}}(y) = \frac{y + 4}{3}$.
Replacing $y$ with $x$,we get ${f^{ - 1}}(x) = \frac{x + 4}{3}$.
10
EasyMCQ
If $f(x) = \frac{x}{1 + x}$,then ${f^{-1}}(x)$ is equal to
A
$\frac{1 + x}{x}$
B
$\frac{1}{1 + x}$
C
$\frac{1 + x}{1 - x}$
D
$\frac{x}{1 - x}$

Solution

(D) Given $f(x) = \frac{x}{1 + x}$.
Let $y = f(x)$,then $x = {f^{-1}}(y)$.
Substituting $y$ for $f(x)$,we have $y = \frac{x}{1 + x}$.
Cross-multiplying gives $y(1 + x) = x$,which simplifies to $y + yx = x$.
Rearranging the terms to solve for $x$,we get $y = x - yx = x(1 - y)$.
Thus,$x = \frac{y}{1 - y}$.
Since $x = {f^{-1}}(y)$,we have ${f^{-1}}(y) = \frac{y}{1 - y}$.
Replacing $y$ with $x$,we obtain ${f^{-1}}(x) = \frac{x}{1 - x}$.
11
MediumMCQ
Which of the following functions is an invertible function?
A
$f(x) = \frac{1}{x - 1}$
B
$f(x) = x^2$ for all $x$
C
$f(x) = x^2, x \ge 0$
D
$f(x) = x^2, x \le 0$

Solution

(A) function is invertible if and only if it is a bijection (both one-to-one and onto).
$1$. For $f(x) = \frac{1}{x - 1}$,the domain is $x \in \mathbb{R} \setminus \{1\}$ and the range is $y \in \mathbb{R} \setminus \{0\}$. This function is one-to-one and onto,hence it is invertible.
$2$. For $f(x) = x^2$ defined for all $x \in \mathbb{R}$,it is not one-to-one because $f(1) = f(-1) = 1$. Thus,it is not invertible.
$3$. For $f(x) = x^2$ with $x \ge 0$ or $x \le 0$,the function becomes a bijection on its restricted domain and codomain,but the standard definition of an invertible function usually refers to the function being bijective over its natural domain. However,among the given choices,$f(x) = \frac{1}{x - 1}$ is the most standard example of a function that is bijective on its entire domain.
12
MediumMCQ
The inverse of the function $f(x) = \frac{10^x - 10^{-x}}{10^x + 10^{-x}}$ is
A
$\frac{1}{2} \log_{10} \left( \frac{1+x}{1-x} \right)$
B
$\frac{1}{2} \log_{10} \left( \frac{1-x}{1+x} \right)$
C
$\frac{1}{4} \log_{10} \left( \frac{2x}{2-x} \right)$
D
None of these

Solution

(A) Let $y = \frac{10^x - 10^{-x}}{10^x + 10^{-x}}$.
To find the inverse,we solve for $x$ in terms of $y$.
Multiply the numerator and denominator by $10^x$:
$y = \frac{10^{2x} - 1}{10^{2x} + 1}$.
$y(10^{2x} + 1) = 10^{2x} - 1$.
$y \cdot 10^{2x} + y = 10^{2x} - 1$.
$1 + y = 10^{2x}(1 - y)$.
$10^{2x} = \frac{1+y}{1-y}$.
Taking $\log_{10}$ on both sides:
$2x = \log_{10} \left( \frac{1+y}{1-y} \right)$.
$x = \frac{1}{2} \log_{10} \left( \frac{1+y}{1-y} \right)$.
Replacing $y$ with $x$ to get the inverse function:
$f^{-1}(x) = \frac{1}{2} \log_{10} \left( \frac{1+x}{1-x} \right)$.
13
EasyMCQ
Inverse of the function $y = 2x - 3$ is
A
$\frac{x + 3}{2}$
B
$\frac{x - 3}{2}$
C
$\frac{1}{2x - 3}$
D
None of these

Solution

(A) Given the function $y = 2x - 3$.
To find the inverse,we solve for $x$ in terms of $y$:
$y + 3 = 2x$
$x = \frac{y + 3}{2}$
Now,replace $y$ with $x$ to express the inverse function $f^{-1}(x)$:
$f^{-1}(x) = \frac{x + 3}{2}$
Thus,the correct option is $A$.
14
EasyMCQ
Let the function $f$ be defined by $f(x) = \frac{2x + 1}{1 - 3x}$,then $f^{-1}(x)$ is
A
$\frac{x - 1}{3x + 2}$
B
$\frac{3x + 2}{x - 1}$
C
$\frac{x + 1}{3x - 2}$
D
$\frac{2x + 1}{1 - 3x}$

Solution

(A) To find the inverse of the function $f(x) = \frac{2x + 1}{1 - 3x}$,we set $y = f(x)$.
$y = \frac{2x + 1}{1 - 3x}$
Multiply both sides by $(1 - 3x)$:
$y(1 - 3x) = 2x + 1$
$y - 3xy = 2x + 1$
Rearrange the terms to isolate $x$:
$y - 1 = 2x + 3xy$
$y - 1 = x(2 + 3y)$
$x = \frac{y - 1}{3y + 2}$
Since $f^{-1}(y) = x$,we have $f^{-1}(y) = \frac{y - 1}{3y + 2}$.
Replacing $y$ with $x$,we get $f^{-1}(x) = \frac{x - 1}{3x + 2}$.
15
EasyMCQ
If $f(x) = x^2 + 1$,then $f^{-1}(17)$ and $f^{-1}(-3)$ will be
A
$4, 1$
B
$4, 0$
C
$3, 2$
D
None of these

Solution

(D) Given $f(x) = x^2 + 1$.
To find the inverse,let $y = f(x) = x^2 + 1$.
Solving for $x$,we get $x^2 = y - 1$,which implies $x = \pm \sqrt{y - 1}$.
Thus,$f^{-1}(y) = \pm \sqrt{y - 1}$.
For $f^{-1}(17)$,we substitute $y = 17$: $f^{-1}(17) = \pm \sqrt{17 - 1} = \pm \sqrt{16} = \pm 4$.
For $f^{-1}(-3)$,we substitute $y = -3$: $f^{-1}(-3) = \pm \sqrt{-3 - 1} = \pm \sqrt{-4}$.
Since $\sqrt{-4}$ is not a real number,$f^{-1}(-3)$ is not defined in the set of real numbers.
Therefore,the correct option is $D$ (None of these).
16
EasyMCQ
Let $f(x) = \sin x + \cos x$ and $g(x) = x^2 - 1$. Then $g(f(x))$ is invertible for $x \in $
A
$[ - \frac{\pi }{2}, 0 ]$
B
$[ - \frac{\pi }{2}, \pi ]$
C
$[ - \frac{\pi }{4}, \frac{\pi }{4} ]$
D
$[ 0, \frac{\pi }{2} ]$

Solution

(C) Given $f(x) = \sin x + \cos x$ and $g(x) = x^2 - 1$.
First,we find the composition $g(f(x))$:
$g(f(x)) = (\sin x + \cos x)^2 - 1$
$= (\sin^2 x + \cos^2 x + 2 \sin x \cos x) - 1$
$= (1 + \sin 2x) - 1 = \sin 2x$.
For a function to be invertible,it must be bijective (one-to-one and onto).
The function $h(x) = \sin 2x$ is invertible in the interval where the argument $2x$ lies within the principal branch of the inverse sine function,i.e.,$[ -\frac{\pi}{2}, \frac{\pi}{2} ]$.
Thus,we require:
$-\frac{\pi}{2} \le 2x \le \frac{\pi}{2}$
Dividing by $2$,we get:
$-\frac{\pi}{4} \le x \le \frac{\pi}{4}$.
Therefore,$g(f(x))$ is invertible for $x \in [ -\frac{\pi}{4}, \frac{\pi}{4} ]$.
17
EasyMCQ
If $f(x) = \frac{2x - 1}{x + 5}$ $(x \ne -5)$,then $f^{-1}(x)$ is equal to
A
$\frac{x + 5}{2x - 1}, \; x \ne \frac{1}{2}$
B
$\frac{5x + 1}{2 - x}, \; x \ne 2$
C
$\frac{5x - 1}{2 - x}, \; x \ne 2$
D
$\frac{x - 5}{2x + 1}, \; x \ne \frac{1}{2}$

Solution

(B) Let $f(x) = y$,which implies $x = f^{-1}(y)$.
Given $y = \frac{2x - 1}{x + 5}$,where $x \ne -5$.
Multiply both sides by $(x + 5)$:
$y(x + 5) = 2x - 1$
$xy + 5y = 2x - 1$
Rearrange the terms to isolate $x$:
$5y + 1 = 2x - xy$
$5y + 1 = x(2 - y)$
Solving for $x$:
$x = \frac{5y + 1}{2 - y}$
Since $x = f^{-1}(y)$,we have:
$f^{-1}(y) = \frac{5y + 1}{2 - y}$
Replacing $y$ with $x$,we get:
$f^{-1}(x) = \frac{5x + 1}{2 - x}$,where $x \ne 2$.
18
MediumMCQ
$A$ condition for a function $y = f(x)$ to have an inverse is that it should be
A
Defined for all $x$
B
Continuous everywhere
C
Strictly monotonic and continuous in the domain
D
An even function

Solution

(C) For a function $f: A \to B$ to have an inverse,it must be a bijection,which means it must be both one-to-one (injective) and onto (surjective).
$A$ function that is strictly monotonic (either strictly increasing or strictly decreasing) is always one-to-one.
If a function is continuous and strictly monotonic on its domain,it maps its domain to its range in a one-to-one manner,ensuring the existence of an inverse function $f^{-1}$.
19
MediumMCQ
If ${e^x} = y + \sqrt {1 + {y^2}} $,then $y =$
A
$\frac{{{e^x} + {e^{ - x}}}}{2}$
B
$\frac{{{e^x} - {e^{ - x}}}}{2}$
C
${e^x} + {e^{ - x}}$
D
${e^x} - {e^{ - x}}$

Solution

(B) Given that ${e^x} = y + \sqrt{1 + y^2}$.
Rearranging the terms,we get ${e^x} - y = \sqrt{1 + y^2}$.
Squaring both sides,we obtain $({e^x} - y)^2 = 1 + y^2$.
Expanding the left side,${e^{2x}} + y^2 - 2y{e^x} = 1 + y^2$.
Subtracting $y^2$ from both sides,${e^{2x}} - 2y{e^x} = 1$.
Rearranging to solve for $y$,$2y{e^x} = {e^{2x}} - 1$.
Dividing by $2{e^x}$,$y = \frac{{e^{2x}} - 1}{2{e^x}}$.
Simplifying the expression,$y = \frac{1}{2} \left( \frac{e^{2x}}{e^x} - \frac{1}{e^x} \right) = \frac{e^x - e^{-x}}{2}$.
20
EasyMCQ
Let $f:(2, 3) \to (0, 1)$ be defined by $f(x) = x - [x]$. Then ${f^{ - 1}}(x)$ equals:
A
$x - 2$
B
$x + 1$
C
$x - 1$
D
$x + 2$

Solution

(D) Given the function $f:(2, 3) \to (0, 1)$ defined by $f(x) = x - [x]$.
For $x \in (2, 3)$,the greatest integer function $[x] = 2$.
Therefore,$f(x) = x - 2$.
Let $y = f(x)$,so $y = x - 2$.
To find the inverse,solve for $x$ in terms of $y$:
$x = y + 2$.
Replacing $y$ with $x$,we get ${f^{ - 1}}(x) = x + 2$.
21
DifficultMCQ
Let $f(x) = (x + 1)^2 - 1$ for $x \ge -1$. Then the set $S = \{ x : f(x) = f^{-1}(x) \}$ is
A
Empty
B
$\{0, -1\}$
C
$\{0, 1, -1\}$
D
$\{0, -1, \frac{-3 + i\sqrt{3}}{2}, \frac{-3 - i\sqrt{3}}{2}\}$

Solution

(D) Given $f(x) = (x + 1)^2 - 1$ for $x \ge -1$.
To find the set $S = \{ x : f(x) = f^{-1}(x) \}$,we note that the solutions to $f(x) = f^{-1}(x)$ are the same as the solutions to $f(x) = x$ and the points where the graph of $f(x)$ intersects its inverse.
Setting $f(x) = x$,we get $(x + 1)^2 - 1 = x$.
$x^2 + 2x + 1 - 1 = x \Rightarrow x^2 + x = 0 \Rightarrow x(x + 1) = 0$.
Thus,$x = 0$ and $x = -1$ are solutions.
However,the equation $f(x) = f^{-1}(x)$ is equivalent to $f(f(x)) = x$ for $x$ in the domain.
$( (x + 1)^2 - 1 + 1 )^2 - 1 = x \Rightarrow ((x + 1)^2)^2 - 1 = x$.
$(x + 1)^4 - 1 = x \Rightarrow (x + 1)^4 - (x + 1) = 0$.
$(x + 1) [ (x + 1)^3 - 1 ] = 0$.
This gives $x + 1 = 0 \Rightarrow x = -1$ or $(x + 1)^3 = 1$.
The roots of $(x + 1)^3 = 1$ are $x + 1 = 1, \omega, \omega^2$,where $\omega = \frac{-1 + i\sqrt{3}}{2}$.
$x + 1 = 1 \Rightarrow x = 0$.
$x + 1 = \frac{-1 + i\sqrt{3}}{2} \Rightarrow x = \frac{-3 + i\sqrt{3}}{2}$.
$x + 1 = \frac{-1 - i\sqrt{3}}{2} \Rightarrow x = \frac{-3 - i\sqrt{3}}{2}$.
Thus,the set $S = \{ 0, -1, \frac{-3 + i\sqrt{3}}{2}, \frac{-3 - i\sqrt{3}}{2} \}$.
22
DifficultMCQ
Let $g(x)$ be the inverse of an invertible function $f(x)$ which is differentiable at $x = c$. Then $g'(f(c))$ equals:
A
$f'(c)$
B
$\frac{1}{f'(c)}$
C
$f(c)$
D
None of these

Solution

(B) Since $g(x)$ is the inverse of the function $f(x)$,we have $g(f(x)) = x$ for all $x$ in the domain.
Differentiating both sides with respect to $x$ using the chain rule:
$\frac{d}{dx}[g(f(x))] = \frac{d}{dx}(x)$
$g'(f(x)) \cdot f'(x) = 1$
Assuming $f'(x) \neq 0$,we can write:
$g'(f(x)) = \frac{1}{f'(x)}$
Substituting $x = c$ into the equation:
$g'(f(c)) = \frac{1}{f'(c)}$
Thus,the correct option is $B$.
23
DifficultMCQ
Let $g(x)$ be the inverse of the function $f(x)$ and $f'(x) = \frac{1}{1 + x^3}$. Then $g'(x)$ is equal to
A
$\frac{1}{1 + (g(x))^3}$
B
$\frac{1}{1 + (f(x))^3}$
C
$1 + (g(x))^3$
D
$1 + (f(x))^3$

Solution

(C) Given that $g(x)$ is the inverse of $f(x)$,we have $f(g(x)) = x$.
Differentiating both sides with respect to $x$ using the chain rule:
$f'(g(x)) \cdot g'(x) = 1$
Therefore,$g'(x) = \frac{1}{f'(g(x))}$.
Given $f'(x) = \frac{1}{1 + x^3}$,we substitute $g(x)$ for $x$ in the derivative expression:
$f'(g(x)) = \frac{1}{1 + (g(x))^3}$.
Substituting this back into the expression for $g'(x)$:
$g'(x) = \frac{1}{\frac{1}{1 + (g(x))^3}} = 1 + (g(x))^3$.
Thus,the correct option is $C$.
24
MediumMCQ
The condition for a function $y = f(x)$ to be invertible is that it must be:
A
Defined for all $x$
B
Continuous everywhere
C
Strictly monotonic and continuous in its domain
D
An even function

Solution

(C) function $f: A \to B$ is invertible if and only if it is a bijection,meaning it is both one-to-one (injective) and onto (surjective).
For a real-valued function $f(x)$ to be a bijection on its domain,it must be strictly monotonic (either strictly increasing or strictly decreasing) and continuous.
If a function is strictly monotonic,it is guaranteed to be one-to-one.
If it is also continuous on its domain,it maps the domain to its range,satisfying the onto condition.
Therefore,the correct condition is that the function must be strictly monotonic and continuous in its domain.
25
MediumMCQ
If $g$ is the inverse of a function $f$ and $f'(x) = \frac{1}{1 + x^5}$,then $g'(x)$ is equal to:
A
$1 + (g(x))^5$
B
$\frac{1}{1 + (g(x))^5}$
C
$1 + x^5$
D
$5x^4$

Solution

(A) Given that $g$ is the inverse of $f$,we have $f(g(x)) = x$.
Differentiating both sides with respect to $x$ using the chain rule,we get $f'(g(x)) \cdot g'(x) = 1$.
Therefore,$g'(x) = \frac{1}{f'(g(x))}$.
Given $f'(x) = \frac{1}{1 + x^5}$,substituting $g(x)$ for $x$ gives $f'(g(x)) = \frac{1}{1 + (g(x))^5}$.
Thus,$g'(x) = \frac{1}{\frac{1}{1 + (g(x))^5}} = 1 + (g(x))^5$.
26
EasyMCQ
Let $f: N \to Y$ be a function defined as $f(x) = 4x + 3$,where $Y = \{y \in N : y = 4x + 3, x \in N\}$. Show that $f$ is invertible and find its inverse.
A
$g(y) = \frac{3y + 4}{3}$
B
$g(y) = 4 + \frac{y + 3}{4}$
C
$g(y) = \frac{y + 3}{4}$
D
$g(y) = \frac{y - 3}{4}$

Solution

(D) To show that $f$ is invertible,we need to find a function $g: Y \to N$ such that $g \circ f = I_N$ and $f \circ g = I_Y$.
Given $f(x) = 4x + 3$,let $y = f(x) = 4x + 3$.
Solving for $x$ in terms of $y$,we get $y - 3 = 4x$,which implies $x = \frac{y - 3}{4}$.
Define a function $g: Y \to N$ by $g(y) = \frac{y - 3}{4}$.
Now,check the composition $g \circ f(x) = g(f(x)) = g(4x + 3) = \frac{(4x + 3) - 3}{4} = \frac{4x}{4} = x = I_N(x)$.
Next,check the composition $f \circ g(y) = f(g(y)) = f\left(\frac{y - 3}{4}\right) = 4\left(\frac{y - 3}{4}\right) + 3 = (y - 3) + 3 = y = I_Y(y)$.
Since $g \circ f = I_N$ and $f \circ g = I_Y$,the function $f$ is invertible and its inverse is $g(y) = \frac{y - 3}{4}$.
27
MediumMCQ
Let $f(x)=(x+1)^2-1$ for $x \geq -1$.
Statement-$1$: $S=\{x:f(x)=f^{-1}(x)\}=\{0, -1\}$
Statement-$2$: $f$ is a bijection.
A
Statement-$1$ is true,Statement-$2$ is true; Statement-$2$ is a correct explanation for Statement-$1$
B
Statement-$1$ is true,Statement-$2$ is true; Statement-$2$ is not a correct explanation for Statement-$1$
C
Statement-$1$ is false,Statement-$2$ is true;
D
Statement-$1$ is true,Statement-$2$ is false

Solution

(A) Given $f(x) = (x+1)^2 - 1$ for $x \geq -1$.
To find $f^{-1}(x)$,set $y = (x+1)^2 - 1$. Since $x \geq -1$,$y \geq -1$.
Solving for $x$: $(x+1)^2 = y+1 \Rightarrow x+1 = \sqrt{y+1} \Rightarrow x = \sqrt{y+1} - 1$.
Thus,$f^{-1}(x) = \sqrt{x+1} - 1$. Since $f$ is strictly increasing on $[-1, \infty)$,it is a bijection (one-one and onto). So,Statement-$2$ is true.
To find $S = \{x : f(x) = f^{-1}(x)\}$,we solve $f(x) = x$ because $f$ is increasing.
$(x+1)^2 - 1 = x \Rightarrow x^2 + 2x + 1 - 1 = x \Rightarrow x^2 + x = 0 \Rightarrow x(x+1) = 0$.
Thus,$x = 0$ or $x = -1$. So,$S = \{0, -1\}$. Statement-$1$ is true.
Since $f(x) = f^{-1}(x)$ is equivalent to $f(x) = x$ for increasing functions,Statement-$2$ explains Statement-$1$.
28
DifficultMCQ
Let $f(x) = (x - 1)^2 + 1$ for $x \ge 1$.
Statement-$1$: $S = \{x : f(x) = f^{-1}(x)\} = \{1, 2\}$.
Statement-$2$: $f$ is a bijection and $f^{-1}(x) = 1 + \sqrt{x - 1}$ for $x \ge 1$.
A
Statement-$1$ is true,Statement-$2$ is false.
B
Statement-$1$ is false,Statement-$2$ is true.
C
Statement-$1$ is true,Statement-$2$ is true; Statement-$2$ is not a correct explanation for Statement-$1$.
D
Statement-$1$ is true,Statement-$2$ is true; Statement-$2$ is a correct explanation for Statement-$1$.

Solution

(D) Given $f(x) = (x - 1)^2 + 1$ for $x \ge 1$.
To find $f^{-1}(x)$,set $y = (x - 1)^2 + 1$.
Then $y - 1 = (x - 1)^2$. Since $x \ge 1$,we have $x - 1 = \sqrt{y - 1}$,so $x = 1 + \sqrt{y - 1}$.
Thus,$f^{-1}(x) = 1 + \sqrt{x - 1}$ for $x \ge 1$. So,Statement-$2$ is true.
To find $S = \{x : f(x) = f^{-1}(x)\}$,we solve $f(x) = x$.
$(x - 1)^2 + 1 = x \implies x^2 - 2x + 1 + 1 = x \implies x^2 - 3x + 2 = 0$.
$(x - 1)(x - 2) = 0$,so $x = 1$ or $x = 2$.
Since both values satisfy $x \ge 1$,$S = \{1, 2\}$. So,Statement-$1$ is true.
Since $f(x) = f^{-1}(x)$ is equivalent to $f(x) = x$ for increasing functions,Statement-$2$ explains Statement-$1$.
29
DifficultMCQ
Suppose $f(x) = (x + 1)^2$ for $x \ge -1$. If $g(x)$ is the function whose graph is the reflection of the graph of $f(x)$ with respect to the line $y = x$,then $g(x)$ equals
A
$-\sqrt{x} - 1, x \ge 0$
B
$\frac{1}{(x + 1)^2}, x > -1$
C
$\sqrt{x + 1}, x \ge -1$
D
$\sqrt{x} - 1, x \ge 0$

Solution

(D) The graph of $f(x)$ is given by the equation $y = (x + 1)^2$ for $x \ge -1$.
To find the reflection of the graph of $f(x)$ with respect to the line $y = x$,we interchange $x$ and $y$.
Thus,the equation for the graph of $g(x)$ becomes $x = (y + 1)^2$ for $y \ge -1$.
Solving for $y$:
$y + 1 = \pm\sqrt{x}$
Since $y \ge -1$,we have $y + 1 \ge 0$,so we take the positive root:
$y + 1 = \sqrt{x}$
$y = \sqrt{x} - 1$
Since the domain of $f(x)$ is $x \ge -1$ and its range is $y \ge 0$,the domain of $g(x)$ is $x \ge 0$ and its range is $y \ge -1$.
Therefore,$g(x) = \sqrt{x} - 1$ for $x \ge 0$.
Solution diagram
30
MediumMCQ
If $f:[1, +\infty) \to [2, +\infty)$ is given by $f(x) = x + \frac{1}{x}$,then ${f^{-1}}(x)$ equals:
A
$\frac{x + \sqrt{x^2 - 4}}{2}$
B
$\frac{x}{1 + x^2}$
C
$\frac{x - \sqrt{x^2 - 4}}{2}$
D
$1 + \sqrt{x^2 - 4}$

Solution

(A) Given $y = f(x) = x + \frac{1}{x}$ where $x \ge 1$ and $y \ge 2$.
Multiplying by $x$,we get $yx = x^2 + 1$,which rearranges to the quadratic equation $x^2 - yx + 1 = 0$.
Using the quadratic formula $x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$,we get $x = \frac{y \pm \sqrt{y^2 - 4}}{2}$.
Since the domain of $f$ is $[1, +\infty)$,we must have $x \ge 1$.
If we take the negative sign,$x = \frac{y - \sqrt{y^2 - 4}}{2}$. For $y \ge 2$,this value is $\le 1$. Specifically,as $y \to \infty$,this expression approaches $0$.
Thus,to satisfy $x \ge 1$,we must take the positive sign: $x = \frac{y + \sqrt{y^2 - 4}}{2}$.
Replacing $y$ with $x$,we obtain ${f^{-1}}(x) = \frac{x + \sqrt{x^2 - 4}}{2}$.
31
EasyMCQ
Let $A = \{1, 2, 3\}, B = \{1, 3, 5\}$. $A$ relation $R: A \to B$ is defined by $R = \{(1, 3), (1, 5), (2, 1)\}$. Then ${R^{-1}}$ is:
A
$\{(1, 2), (3, 1), (1, 3), (1, 5)\}$
B
$\{(1, 2), (3, 1), (2, 1)\}$
C
$\{(3, 1), (5, 1), (1, 2)\}$
D
None of these

Solution

(C) The inverse relation ${R^{-1}}$ is defined as the set of all ordered pairs $(y, x)$ such that $(x, y) \in R$.
Given $R = \{(1, 3), (1, 5), (2, 1)\}$.
By swapping the elements of each ordered pair in $R$,we get:
$(1, 3) \to (3, 1)$
$(1, 5) \to (5, 1)$
$(2, 1) \to (1, 2)$
Therefore,${R^{-1}} = \{(3, 1), (5, 1), (1, 2)\}$.
32
EasyMCQ
The relation $R$ on the set of natural numbers is defined by $R = \{(a, b) : a = 2b\}$. Then ${R^{-1}}$ is:
A
$\{(2, 1), (4, 2), (6, 3), \dots\}$
B
$\{(1, 2), (2, 4), (3, 6), \dots\}$
C
${R^{-1}}$ is not defined
D
None of these

Solution

(B) Given the relation $R = \{(a, b) : a = 2b\}$ where $a, b \in \mathbb{N}$.
By substituting values for $b$,we get $R = \{(2, 1), (4, 2), (6, 3), \dots\}$.
The inverse relation ${R^{-1}}$ is obtained by interchanging the elements of the ordered pairs in $R$.
Thus,${R^{-1}} = \{(b, a) : (a, b) \in R\} = \{(1, 2), (2, 4), (3, 6), \dots\}$.
33
EasyMCQ
Let $A = \{1, 2, 3\}$ and $B = \{1, 3, 5\}$. If a relation $R$ is defined from $A$ to $B$ as $R = \{(1, 3), (2, 5), (3, 3)\}$,then find $R^{-1}$.
A
$\{(3, 3), (3, 1), (5, 2)\}$
B
$\{(1, 3), (2, 5), (3, 3)\}$
C
$\{(1, 3), (5, 2)\}$
D
None of these

Solution

(A) The relation $R$ is given by $R = \{(1, 3), (2, 5), (3, 3)\}$.
By definition,the inverse relation $R^{-1}$ is obtained by interchanging the elements of the ordered pairs in $R$.
That is,if $(a, b) \in R$,then $(b, a) \in R^{-1}$.
Applying this to each element of $R$:
$(1, 3) \in R \implies (3, 1) \in R^{-1}$
$(2, 5) \in R \implies (5, 2) \in R^{-1}$
$(3, 3) \in R \implies (3, 3) \in R^{-1}$
Therefore,$R^{-1} = \{(3, 1), (5, 2), (3, 3)\}$.
Rearranging the set,we get $R^{-1} = \{(3, 3), (3, 1), (5, 2)\}$.
Thus,the correct option is $A$.
34
AdvancedMCQ
Let $f(x) = \int\limits_2^x \frac{dt}{\sqrt{1 + t^4}}$ and $g$ be the inverse of $f$. Then the value of $g'(0)$ is
A
$1$
B
$17$
C
$\sqrt{17}$
D
none of these

Solution

(C) Given $f(x) = \int\limits_2^x \frac{dt}{\sqrt{1 + t^4}}$.
By the Fundamental Theorem of Calculus,$f'(x) = \frac{1}{\sqrt{1 + x^4}}$.
Since $g$ is the inverse of $f$,we have $g(f(x)) = x$. Differentiating both sides with respect to $x$,we get $g'(f(x)) \cdot f'(x) = 1$,which implies $g'(f(x)) = \frac{1}{f'(x)} = \sqrt{1 + x^4}$.
To find $g'(0)$,we need to find $x$ such that $f(x) = 0$.
$f(x) = \int\limits_2^x \frac{dt}{\sqrt{1 + t^4}} = 0$ implies $x = 2$.
Therefore,$g'(0) = g'(f(2)) = \sqrt{1 + 2^4} = \sqrt{1 + 16} = \sqrt{17}$.
35
DifficultMCQ
If $g$ is the inverse of $f$ and $f'(x) = \frac{1}{1 + x^5}$,then $g'(x) =$
A
$1 + [g(x)]^5$
B
$\frac{1}{1 + [g(x)]^5}$
C
$-\frac{1}{1 + [g(x)]^5}$
D
None of these

Solution

(A) Given that $g$ is the inverse of $f$,we have $f(g(x)) = x$.
Differentiating both sides with respect to $x$ using the chain rule:
$f'(g(x)) \cdot g'(x) = 1$
We are given $f'(x) = \frac{1}{1 + x^5}$. Therefore,$f'(g(x)) = \frac{1}{1 + [g(x)]^5}$.
Substituting this into the differentiated equation:
$\frac{1}{1 + [g(x)]^5} \cdot g'(x) = 1$
Solving for $g'(x)$:
$g'(x) = 1 + [g(x)]^5$.
36
AdvancedMCQ
Let $g$ be the inverse function of $f$ and $f'(x) = \frac{x^{10}}{1 + x^2}$. If $g(2) = a$,then $g'(2)$ is equal to:
A
$\frac{5}{a^{10}}$
B
$\frac{1 + a^2}{a^{10}}$
C
$\frac{a^{10}}{1 + a^2}$
D
$\frac{1 + a^{10}}{a^2}$

Solution

(B) Since $g$ is the inverse of $f$,we have $f(g(x)) = x$.
Differentiating both sides with respect to $x$ using the chain rule,we get $f'(g(x)) \cdot g'(x) = 1$.
Substituting $x = 2$,we get $f'(g(2)) \cdot g'(2) = 1$.
Given $g(2) = a$,this becomes $f'(a) \cdot g'(2) = 1$,which implies $g'(2) = \frac{1}{f'(a)}$.
We are given $f'(x) = \frac{x^{10}}{1 + x^2}$,so $f'(a) = \frac{a^{10}}{1 + a^2}$.
Therefore,$g'(2) = \frac{1}{\frac{a^{10}}{1 + a^2}} = \frac{1 + a^2}{a^{10}}$.
37
AdvancedMCQ
Let $e^{f(x)} = \ln x$. If $g(x)$ is the inverse function of $f(x)$,then $g'(x)$ is equal to:
A
$e^x$
B
$e^x + x$
C
$e^{(x + e^x)}$
D
$e^{(x + \ln x)}$

Solution

(C) Given $e^{f(x)} = \ln x$. Taking the natural logarithm on both sides,we get $f(x) = \ln(\ln x)$.
Since $g(x)$ is the inverse function of $f(x)$,we have $f(g(x)) = x$.
Substituting $g(x)$ into the expression for $f$,we get $\ln(\ln(g(x))) = x$.
Exponentiating both sides,we get $\ln(g(x)) = e^x$.
Exponentiating again,we get $g(x) = e^{e^x}$.
Now,we differentiate $g(x)$ with respect to $x$ using the chain rule:
$g'(x) = \frac{d}{dx}(e^{e^x}) = e^{e^x} \cdot \frac{d}{dx}(e^x) = e^{e^x} \cdot e^x$.
Using the property of exponents $a^m \cdot a^n = a^{m+n}$,we get $g'(x) = e^{e^x + x}$.
Thus,the correct option is $C$.
38
AdvancedMCQ
The function $f(x) = e^x + x$,being differentiable and one-to-one,has a differentiable inverse $f^{-1}(x)$. The value of $(f^{-1})'(f(\ln 2))$ is
A
$\frac{1}{\ln 2}$
B
$\frac{1}{3}$
C
$\frac{1}{4}$
D
None of these

Solution

(B) Given $f(x) = e^x + x$.
We need to find $(f^{-1})'(f(\ln 2))$.
By the formula for the derivative of an inverse function,$(f^{-1})'(y) = \frac{1}{f'(x)}$,where $y = f(x)$.
Here,$y = f(\ln 2)$,so $x = \ln 2$.
First,find $f'(x)$:
$f'(x) = \frac{d}{dx}(e^x + x) = e^x + 1$.
Now,evaluate $f'(x)$ at $x = \ln 2$:
$f'(\ln 2) = e^{\ln 2} + 1 = 2 + 1 = 3$.
Therefore,$(f^{-1})'(f(\ln 2)) = \frac{1}{f'(\ln 2)} = \frac{1}{3}$.
39
AdvancedMCQ
Let $f(x) = x^3 + 8x + 3$. Which one of the properties of the derivative enables you to conclude that $f(x)$ has an inverse?
A
$f'(x)$ is a polynomial of even degree.
B
$f'(x)$ is self-inverse.
C
The domain of $f'(x)$ is the range of $f'(x)$.
D
$f'(x)$ is always positive.

Solution

(D) To determine if a function $f(x)$ has an inverse,we check if it is strictly monotonic (either strictly increasing or strictly decreasing).
Given $f(x) = x^3 + 8x + 3$.
The derivative is $f'(x) = \frac{d}{dx}(x^3 + 8x + 3) = 3x^2 + 8$.
Since $x^2 \ge 0$ for all real $x$,it follows that $3x^2 \ge 0$.
Therefore,$f'(x) = 3x^2 + 8 \ge 8$.
Since $f'(x) > 0$ for all $x \in \mathbb{R}$,the function $f(x)$ is strictly increasing on its entire domain.
$A$ strictly monotonic function is always one-to-one (injective) and thus possesses an inverse.
Therefore,the property that $f'(x)$ is always positive allows us to conclude that $f(x)$ has an inverse.
40
AdvancedMCQ
Let $f : R \rightarrow R$ be defined by $f(x) = x^3 - 3x^2 + 3x - 2$. Then $f^{-1}(x)$ is given by:
A
$1 + \sqrt[3]{x + 1}$
B
$1 + \sqrt[3]{x - 1}$
C
$\sqrt[3]{x + 1} - 1$
D
$\sqrt[3]{x - 1} - 1$

Solution

(A) Given the function $f(x) = x^3 - 3x^2 + 3x - 2$.
We can rewrite the function as $f(x) = (x^3 - 3x^2 + 3x - 1) - 1$.
This simplifies to $f(x) = (x - 1)^3 - 1$.
To find the inverse $f^{-1}(x)$,let $y = f(x)$.
So,$y = (x - 1)^3 - 1$.
Add $1$ to both sides: $y + 1 = (x - 1)^3$.
Take the cube root of both sides: $\sqrt[3]{y + 1} = x - 1$.
Add $1$ to both sides: $x = 1 + \sqrt[3]{y + 1}$.
Replacing $y$ with $x$,we get $f^{-1}(x) = 1 + \sqrt[3]{x + 1}$.
41
AdvancedMCQ
If $f(x) = x^{11} + \sin^3(35x) + 111x$,then the value of $f^{-1}(\sin \frac{\pi}{5}) + f^{-1}(\sin \frac{6\pi}{5}) + f^{-1}(\sin \frac{\pi}{7}) + f^{-1}(\sin \frac{8\pi}{7})$ is equal to
A
$f(\pi^{11})$
B
$f(\frac{\pi}{7})^{11}$
C
$f(\frac{\pi}{5})^{11}$
D
$f(0)$

Solution

(D) Given $f(x) = x^{11} + \sin^3(35x) + 111x$.
Since $x^{11}$,$\sin^3(35x)$,and $111x$ are all odd functions,their sum $f(x)$ is an odd function,i.e.,$f(-x) = -f(x)$.
If $f(x)$ is an odd function,then its inverse $f^{-1}(x)$ is also an odd function,i.e.,$f^{-1}(-y) = -f^{-1}(y)$.
We need to calculate $S = f^{-1}(\sin \frac{\pi}{5}) + f^{-1}(\sin \frac{6\pi}{5}) + f^{-1}(\sin \frac{\pi}{7}) + f^{-1}(\sin \frac{8\pi}{7})$.
Note that $\sin \frac{6\pi}{5} = \sin(\pi + \frac{\pi}{5}) = -\sin \frac{\pi}{5}$.
Similarly,$\sin \frac{8\pi}{7} = \sin(\pi + \frac{\pi}{7}) = -\sin \frac{\pi}{7}$.
Substituting these into the expression:
$S = f^{-1}(\sin \frac{\pi}{5}) + f^{-1}(-\sin \frac{\pi}{5}) + f^{-1}(\sin \frac{\pi}{7}) + f^{-1}(-\sin \frac{\pi}{7})$.
Using the property $f^{-1}(-y) = -f^{-1}(y)$:
$S = f^{-1}(\sin \frac{\pi}{5}) - f^{-1}(\sin \frac{\pi}{5}) + f^{-1}(\sin \frac{\pi}{7}) - f^{-1}(\sin \frac{\pi}{7}) = 0$.
Since $f(0) = 0^{11} + \sin^3(0) + 111(0) = 0$,the result $0$ is equal to $f(0)$.
42
AdvancedMCQ
If the function $f(x) = -4e^{\left(\frac{1-x}{2}\right)} + 1 + x + \frac{x^2}{2} + \frac{x^3}{3}$ and $g(x) = f^{-1}(x)$,then the value of $g'(-\frac{7}{6})$ equals
A
$\frac{1}{5}$
B
$-\frac{1}{5}$
C
$\frac{6}{7}$
D
$-\frac{6}{7}$

Solution

(A) Given $f(x) = -4e^{\left(\frac{1-x}{2}\right)} + 1 + x + \frac{x^2}{2} + \frac{x^3}{3}$.
Since $g(x) = f^{-1}(x)$,we have $g(f(x)) = x$.
Differentiating both sides with respect to $x$,we get $g'(f(x)) \cdot f'(x) = 1$,which implies $g'(f(x)) = \frac{1}{f'(x)}$.
First,we find $x$ such that $f(x) = -\frac{7}{6}$. By inspection,if $x = 1$,then $f(1) = -4e^0 + 1 + 1 + \frac{1}{2} + \frac{1}{3} = -4 + 2 + \frac{3+2}{6} = -2 + \frac{5}{6} = -\frac{7}{6}$.
Now,calculate $f'(x) = -4e^{\left(\frac{1-x}{2}\right)} \cdot \left(-\frac{1}{2}\right) + 1 + x + x^2 = 2e^{\left(\frac{1-x}{2}\right)} + 1 + x + x^2$.
At $x = 1$,$f'(1) = 2e^0 + 1 + 1 + 1 = 2 + 3 = 5$.
Therefore,$g'(-\frac{7}{6}) = g'(f(1)) = \frac{1}{f'(1)} = \frac{1}{5}$.
43
AdvancedMCQ
The number of integral values of $a$ for which the function $f: R \to R, f(x) = 2x^3 - 3(a + 2)x^2 + 12ax - 7$ where $a \in [-4, 6]$ is invertible,is
A
$0$
B
$1$
C
$2$
D
$3$

Solution

(B) function $f(x)$ is invertible if and only if it is strictly monotonic (either strictly increasing or strictly decreasing) on its domain.
For a cubic polynomial $f(x)$,this occurs when $f'(x) \ge 0$ or $f'(x) \le 0$ for all $x \in R$.
First,find the derivative: $f'(x) = 6x^2 - 6(a + 2)x + 12a$.
Factor the derivative: $f'(x) = 6(x^2 - (a + 2)x + 2a) = 6(x - a)(x - 2)$.
For $f(x)$ to be monotonic,$f'(x)$ must not change sign. However,since $f'(x)$ is a quadratic with two roots $a$ and $2$,it changes sign unless the roots are equal.
Thus,we must have $a = 2$.
If $a = 2$,$f'(x) = 6(x - 2)^2 \ge 0$ for all $x$,so the function is strictly increasing and thus invertible.
Since $a = 2$ is the only integral value in the interval $[-4, 6]$ that satisfies the condition,the number of such values is $1$.
44
AdvancedMCQ
If $f(x) = (2x - 3\pi)^5 + \frac{4}{3}x + \cos x$ and $g$ is the inverse of $f$,then $g'(2\pi) = ?$
A
$\frac{7}{3}$
B
$\frac{3}{7}$
C
$\frac{30\pi^4 + 4}{3}$
D
$\frac{3}{30\pi^4 + 4}$

Solution

(B) Given $f(x) = (2x - 3\pi)^5 + \frac{4}{3}x + \cos x$.
Since $g$ is the inverse of $f$,we have $g(f(x)) = x$.
Differentiating both sides with respect to $x$,we get $g'(f(x)) \cdot f'(x) = 1$,which implies $g'(f(x)) = \frac{1}{f'(x)}$.
We need to find $g'(2\pi)$. Let $f(x) = 2\pi$.
$(2x - 3\pi)^5 + \frac{4}{3}x + \cos x = 2\pi$.
By observation,if $x = \frac{3\pi}{2}$,then $f(\frac{3\pi}{2}) = (2(\frac{3\pi}{2}) - 3\pi)^5 + \frac{4}{3}(\frac{3\pi}{2}) + \cos(\frac{3\pi}{2}) = (0)^5 + 2\pi + 0 = 2\pi$.
Now,find $f'(x)$: $f'(x) = 5(2x - 3\pi)^4 \cdot 2 + \frac{4}{3} - \sin x = 10(2x - 3\pi)^4 + \frac{4}{3} - \sin x$.
At $x = \frac{3\pi}{2}$,$f'(\frac{3\pi}{2}) = 10(0)^4 + \frac{4}{3} - \sin(\frac{3\pi}{2}) = 0 + \frac{4}{3} - (-1) = \frac{4}{3} + 1 = \frac{7}{3}$.
Therefore,$g'(2\pi) = \frac{1}{f'(\frac{3\pi}{2})} = \frac{1}{7/3} = \frac{3}{7}$.
45
AdvancedMCQ
If $f(x) = \exp(2x^3 + 3x^2 + 6x)$ and $g(x)$ is the inverse function of $f(x)$,then the value of $g'(e^{11})$ is -
A
$\frac{1}{e^{11}}$
B
$\frac{1}{11e^{11}}$
C
$\frac{1}{15e^{11}}$
D
$\frac{1}{18e^{11}}$

Solution

(D) Given $f(x) = e^{2x^3 + 3x^2 + 6x}$.
Since $g(x)$ is the inverse of $f(x)$,we have $g(f(x)) = x$.
Differentiating both sides with respect to $x$,we get $g'(f(x)) \cdot f'(x) = 1$,which implies $g'(f(x)) = \frac{1}{f'(x)}$.
First,find $f'(x)$:
$f'(x) = e^{2x^3 + 3x^2 + 6x} \cdot \frac{d}{dx}(2x^3 + 3x^2 + 6x) = e^{2x^3 + 3x^2 + 6x} \cdot (6x^2 + 6x + 6)$.
We need to find $g'(e^{11})$. Set $f(x) = e^{11}$,so $e^{2x^3 + 3x^2 + 6x} = e^{11}$.
This implies $2x^3 + 3x^2 + 6x = 11$,or $2x^3 + 3x^2 + 6x - 11 = 0$.
By inspection,$x = 1$ is a root because $2(1)^3 + 3(1)^2 + 6(1) - 11 = 2 + 3 + 6 - 11 = 0$.
Thus,$f(1) = e^{11}$.
Using the formula $g'(f(1)) = \frac{1}{f'(1)}$:
$f'(1) = e^{11} \cdot (6(1)^2 + 6(1) + 6) = e^{11} \cdot (6 + 6 + 6) = 18e^{11}$.
Therefore,$g'(e^{11}) = \frac{1}{18e^{11}}$.
46
AdvancedMCQ
Let $f : R \to R$ be defined by $f(x) = \ln(x + \sqrt{x^2 + 1})$. Then the number of solutions of $|f^{-1}(x)| = e^{-|x|}$ is
A
$1$
B
$2$
C
$3$
D
Infinite

Solution

(B) Given $f(x) = \ln(x + \sqrt{x^2 + 1})$.
Let $y = \ln(x + \sqrt{x^2 + 1})$.
Then $e^y = x + \sqrt{x^2 + 1}$.
Also,$e^{-y} = \frac{1}{x + \sqrt{x^2 + 1}} = \sqrt{x^2 + 1} - x$.
Subtracting the two equations: $e^y - e^{-y} = 2x$,which implies $x = \frac{e^y - e^{-y}}{2} = \sinh(y)$.
Thus,$f^{-1}(x) = \sinh(x) = \frac{e^x - e^{-x}}{2}$.
We need to find the number of solutions to $|f^{-1}(x)| = e^{-|x|}$,which is $|sinh(x)| = e^{-|x|}$.
Since both sides are even functions,we can analyze for $x \ge 0$: $\sinh(x) = e^{-x}$.
At $x = 0$,$\sinh(0) = 0$ and $e^0 = 1$,so $0 \neq 1$.
As $x$ increases,$\sinh(x)$ is strictly increasing from $0$ to $\infty$,and $e^{-x}$ is strictly decreasing from $1$ to $0$.
Therefore,there is exactly one intersection point for $x > 0$.
By symmetry,there is also exactly one intersection point for $x < 0$.
Thus,there are $2$ solutions in total.
Solution diagram
47
AdvancedMCQ
Let $f: [4, \infty) \to [1, \infty)$ be a function defined by $f(x) = 5^{x(x - 4)}$. Then $f^{-1}(x)$ is:
A
$2 - \sqrt{4 + \log_5 x}$
B
$2 + \sqrt{4 + \log_5 x}$
C
$(\frac{1}{5})^{x(x - 4)}$
D
$2 + \sqrt{4 - \log_5 x}$

Solution

(B) Given $f(x) = 5^{x(x - 4)} = y$.
Taking $\log_5$ on both sides,we get $\log_5 y = x(x - 4)$.
$\log_5 y = x^2 - 4x$.
To solve for $x$,we complete the square: $x^2 - 4x + 4 = \log_5 y + 4$.
$(x - 2)^2 = \log_5 y + 4$.
$x - 2 = \pm \sqrt{\log_5 y + 4}$.
Since the domain is $x \in [4, \infty)$,we must have $x - 2 \ge 2$,so we take the positive root: $x = 2 + \sqrt{\log_5 y + 4}$.
Replacing $y$ with $x$,we get $f^{-1}(x) = 2 + \sqrt{4 + \log_5 x}$.
48
AdvancedMCQ
If the function $f(x) = x^5 + e^{x/5}$ and $g(x) = f^{-1}(x)$,then the value of $\frac{1}{g'(1 + e^{1/5})}$ is
A
$5 + \frac{e^{1/5}}{5}$
B
$5 + e^{1/5}$
C
$1$
D
$5 + \frac{5}{e}$

Solution

(A) Given $f(x) = x^5 + e^{x/5}$ and $g(x) = f^{-1}(x)$.
We need to find the value of $\frac{1}{g'(1 + e^{1/5})}$.
By the property of inverse functions,$g'(y) = \frac{1}{f'(x)}$ where $y = f(x)$.
Here,$y = 1 + e^{1/5}$.
Setting $f(x) = x^5 + e^{x/5} = 1 + e^{1/5}$,we observe that $x = 1$ satisfies the equation because $1^5 + e^{1/5} = 1 + e^{1/5}$.
Now,find the derivative $f'(x) = \frac{d}{dx}(x^5 + e^{x/5}) = 5x^4 + \frac{1}{5}e^{x/5}$.
At $x = 1$,$f'(1) = 5(1)^4 + \frac{1}{5}e^{1/5} = 5 + \frac{e^{1/5}}{5}$.
Since $g'(y) = \frac{1}{f'(x)}$,we have $g'(1 + e^{1/5}) = \frac{1}{f'(1)} = \frac{1}{5 + \frac{e^{1/5}}{5}}$.
Therefore,$\frac{1}{g'(1 + e^{1/5})} = f'(1) = 5 + \frac{e^{1/5}}{5}$.
49
AdvancedMCQ
If $f(x) = \int\limits_0^x {\frac{1}{{\sqrt {1 + {t^3}} }}\,} dt$ and $h(x)$ is the inverse of $f(x)$,then the value of $\frac{{h''(x)}}{{{h^2}(x)}}$ is
A
$\frac{2}{3}$
B
$\frac{3}{2}$
C
$\frac{3}{4}$
D
$3$

Solution

(B) Given $f(x) = \int\limits_0^x \frac{1}{\sqrt{1+t^3}} dt$. By the Fundamental Theorem of Calculus,$f'(x) = \frac{1}{\sqrt{1+x^3}}$.
Since $h(x)$ is the inverse of $f(x)$,we have $f(h(x)) = x$.
Differentiating both sides with respect to $x$,we get $f'(h(x)) \cdot h'(x) = 1$,which implies $h'(x) = \frac{1}{f'(h(x))} = \sqrt{1 + h^3(x)}$.
Now,differentiate $h'(x) = (1 + h^3(x))^{1/2}$ with respect to $x$:
$h''(x) = \frac{1}{2}(1 + h^3(x))^{-1/2} \cdot (3h^2(x) \cdot h'(x))$.
Substitute $h'(x) = \sqrt{1 + h^3(x)}$ into the expression:
$h''(x) = \frac{3h^2(x) \cdot \sqrt{1 + h^3(x)}}{2 \sqrt{1 + h^3(x)}} = \frac{3}{2} h^2(x)$.
Therefore,$\frac{h''(x)}{h^2(x)} = \frac{3}{2}$.

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