(A) We know that in a triangle,the area $\Delta = rs = \frac{abc}{4R}$.Also,the expression $\frac{1}{ab} + \frac{1}{bc} + \frac{1}{ca}$ can be written

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(A) We know that in a triangle,the area $\Delta = rs = \frac{abc}{4R}$.Also,the expression $\frac{1}{ab} + \frac{1}{bc} + \frac{1}{ca}$ can be written as $\frac{a+b+c}{abc} = \frac{2s}{abc}$.From the area formula,$abc = 4R\Delta = 4R(rs) = 4Rrs$.Substituting this into the expression,we get $\frac{2s}{4Rrs} = \frac{1}{2Rr}$.Given $r=3$ and $R=5$,the value is $\frac{1}{2 \times 5 \times 3} = \frac{1}{30}$.

Class 11 Mathematics Trigonometrical Equations Relation between sides and angles, Solutions of triangles

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In $\triangle ABC$,if $r=3$ and $R=5$,then $\frac{1}{ab} + \frac{1}{bc} + \frac{1}{ca} = $

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A
$\frac{1}{30}$
B
$\frac{12}{15}$
C
$\frac{1}{15}$
D
$\frac{5}{36}$

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In $\triangle ABC$,if $r=3$ and $R=5$,then $\frac{1}{ab} + \frac{1}{bc} + \frac{1}{ca} = $

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