In an $AP$ if $a = -7.2, d = 3.6, a_{n} = 7.2,$ then $n$ is

In an $AP$,if $S_{n} = 3n^{2} + 5n$ and $a_{k} = 164$,find the value of $k$.

The sum of the first $20$ terms of the $A.P.$ $1, 21, 41, \ldots$ is:

Divya deposited $Rs. 1000$ at compound interest at the rate of $10\%$ per annum. The amounts at the end of the first year,second year,third year,$\ldots,$ form an $AP$. Justify your answer.

The sum of the first $n$ terms of an $A.P.$ is given by $S_n = 3n^2 + 5n$. Then,the $n^{th}$ term of the $A.P.$ $T_n = \ldots$

Find the number of terms in the finite $A.P.$ $6, 5.5, 5, \ldots, -12$.