The first term of a $G.P.$ is $1 .$ The sum of the third term and fifth term is $90 .$ Find the common ratio of $G.P.$

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Let $a$ and $r$ be the first term and the common ratio of the $G.P.$ respectively.

$\therefore $ $a=1$         $a_{3}=a r^{2}=r^{2} \quad a_{5}=a r^{4}=r^{4}$

$\therefore r^{2}+r^{4}=90$

$\Rightarrow r^{4}+r^{2}-90=0$

$\Rightarrow r^{2}=\frac{-1+\sqrt{1+360}}{2}=\frac{-1 \pm \sqrt{361}}{2}=-10$ or $9$

$\therefore r=\pm 3$          [ Taking real roots ]

Thus, the common ratio of the $G.P.$ is $±3$ .

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