Question

The ratio of sum of first three terms of a G.P. to the sum of first six terms is 64:91, the common ratio of G.P. is

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

Given $\frac{S_3}{S_6}=\frac{64}{91}=\frac{a(r^3-1)}{a(r^6-1)}$
$\Rightarrow \frac{(r^3-1)}{(r^3+1)(r^3-1)}=\frac{64}{91}$
$\Rightarrow r^3=\frac{27}{64}\therefore r=\frac{3}{4}$