Question

What is the sum of the first five terms of the geometric sequence $5,15,45,...$?

• A. $105$
• B. $305$
• C. $505$
• D. $605$

Answer 1

The correct option is D $605$

Given series is $5,15,45,...$

Here first term $,a=5$ and common ratio $,r=3$
$S_n=\frac{a_1(r^n-1)}{r-1}$
$S_5=\frac{5\left(\right(3{)}^5-1)}{\left(3\right)-1}$
$=\frac{5\left(\right(3{)}^5-1)}{2}$
$=605$