# If S Denotes the Sum of an Infinite G.P. S1 Denotes the Sum of the Squares of Its Terms, Then Prove that the First Term and Common Ratio Are Respectively 2 S S 1 S 2 + S 1 and S 2 − S 1 S 2 + S 1 - Mathematics

If S denotes the sum of an infinite G.P. S1 denotes the sum of the squares of its terms, then prove that the first term and common ratio are respectively

$\frac{2S S_1}{S^2 + S_1}\text { and } \frac{S^2 - S_1}{S^2 + S_1}$

#### Solution

$S = \frac{a}{\left( 1 - r \right)} . . . . . . . (i)$

$\text { And }, S_1 = \frac{a^2}{\left( 1 - r^2 \right)}$

$\Rightarrow S_1 = \frac{a^2}{\left( 1 - r \right)\left( 1 + r \right)} . . . . . . . (ii)$

$\text { Now, putting the value of a in equation (ii) from equation } (i):$

$S_1 = \frac{S^2 \left( 1 - r \right)^2}{\left( 1 - r \right)\left( 1 + r \right)}$

$\Rightarrow S_1 = \frac{S^2 \left( 1 - r \right)}{\left( 1 + r \right)}$

$\Rightarrow S_1 \left( 1 + r \right) = S^2 \left( 1 - r \right)$

$\Rightarrow r\left( S_1 + S^2 \right) = S^2 - S_1$

$\Rightarrow r = \frac{\left( S^2 - S_1 \right)}{\left( S_1 + S^2 \right)}$

$\text { Putting the value of r in equation }(i):$

$\Rightarrow a = S\left( 1 - r \right)$

$\Rightarrow a = S\left( 1 - \frac{\left( S^2 - S_1 \right)}{\left( S_1 + S^2 \right)} \right)$

$\Rightarrow a = S\left( \frac{\left( S_1 + S^2 \right) - \left( S^2 - S_1 \right)}{\left( S_1 + S^2 \right)} \right)$

$\Rightarrow a = \frac{2 {SS}_1}{\left( S_1 + S^2 \right)}$

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#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 20 Geometric Progression
Exercise 20.4 | Q 13 | Page 40