Question

Find the sum of the first 20 terms if the first term is -12 and the common difference is -5 in an arithmetic series.

• A. $-1190$
• B. $-1290$
• C. $-1390$
• D. $-1490$

Answer 1

The correct option is A $-1190$

First term =$a=-12$
$n=20$
$d=-5$
$S_n=\frac{n}{2}[2a+(n-1\left)d\right]$
$S_{20}=\frac{20}{2}[2\times -12+(20-1\left)\right(-5\left)\right]$
$S_{20}=10[-24-95]$
$S_{20}=-1190$