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 $G.P.$

Vedclass pdf generator app on play store
Vedclass iOS app on app store

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 $±3$ .

Similar Questions

Which term of the following sequences:

$\frac{1}{3}, \frac{1}{9}, \frac{1}{27}, \ldots$ is $\frac{1}{19683} ?$

Let $a _1, a _2, a _3, \ldots$ be a $G.P.$ of increasing positive numbers. Let the sum of its $6^{\text {th }}$ and $8^{\text {th }}$ terms be $2$ and the product of its $3^{\text {rd }}$ and $5^{\text {th }}$ terms be $\frac{1}{9}$. Then $6\left( a _2+\right.$ $\left.a_4\right)\left(a_4+a_6\right)$ is equal to

  • [JEE MAIN 2023]

If $a$,$b$,$c \in {R^ + }$ are such that $2a$,$b$ and $4c$ are in $A$.$P$ and $c$,$a$ and $b$ are in $G$.$P$., then

The value of $\overline {0.037} $ where,  $\overline {.037} $ stands for the number $0.037037037........$ is

How many terms of $G.P.$ $3,3^{2}, 3^{3}$... are needed to give the sum $120 ?$