Question

Find the sum of the first $6$ terms of the geometric series $80-20+5+.....$

• A. $63.984$
• B. $32.451$
• C. $54.876$
• D. $25.458$

Answer 1

The correct option is A $63.984$

First term, $a$ is $80$
Common ratio, $r=$ $\frac{-20}{80}=\frac{-1}{4}$
$S_n=\frac{a(1-r^2)}{1-r}$
$S_n=\frac{80(1-(\frac{-1}{4}{)}^2)}{1-\left(\frac{-1}{4}\right)}$
$S_n=\frac{79.98}{1.25}$
$S_n=63.98$