Question:
How many terms of the G.P. $3,3 / 2,3 / 4, \ldots$ be taken together to make $\frac{3069}{512} ?$
Solution:
Here, a = 3
Common ratio, $r=\frac{1}{2}$
$S_{n}=\frac{3069}{512}$
$\therefore S_{n}=3\left\{\frac{1-\left(\frac{1}{2}\right)^{n}}{1-\frac{1}{2}}\right\}$
$\Rightarrow \frac{3069}{512}=3\left\{\frac{1-\frac{1}{2^{n}}}{\frac{1}{2}}\right\}$
$\Rightarrow \frac{3069}{512}=6\left\{1-\frac{1}{2^{n}}\right\}$
$\Rightarrow \frac{3069}{3072}=1-\frac{1}{2^{n}}$
$\Rightarrow \frac{1}{2^{n}}=1-\frac{3069}{3072}$
$\Rightarrow \frac{1}{2^{n}}=\frac{3}{3072}$
$\Rightarrow 2^{n}=\frac{3072}{3}$
$\Rightarrow 2^{n}=1024$
$\Rightarrow 2^{n}=2^{10}$
$\therefore n=10$