Question

Evaluate the sum of the first nine terms of the geometric sequence $5,10,20,...$

• A. $1555$
• B. $2555$
• C. $3555$
• D. $4555$

Answer 1

Answer: (B)

$2555$

Given sequence is $5,10,20,....$

Here $a=5,r=2,n=9$
We know $S_n=\frac{a_1(r^n-1)}{r-1}$
$\Rightarrow S_9=\frac{5(2^9-1)}{2-1}$
$\Rightarrow S_9=\frac{2555}{1}$
$\Rightarrow S_9=2555$