Question

If the first term of a G.P. is 1 and the sum of the third and fifth term is 20, find its common ratio.

• A. 2
• B. -2
• C. 5
• D. -5

Answer 1

The correct option is A

2

First term = 1

$\to a=1$

Let common ratio be r

third term = $r^2$ , fifth term = $r^4$

$r^2$ + $r^4$ = 20 (given)

$r^4$ + $r^2$ - 20 = 0

Solving the equation we get

($r^2$ - 4)($r^2$ + 5) = 0

$r^2$ = 4 ( as $r^2$ cannot be equal to -5)

r = 2