Question

Find the sum of first $8$ terms of the $AP-1,-4,-7,....$

Answer 1

Given,

$a=-1$

$d=-4+1=-3$

$n=8$

$S_n=\frac{n}{2}[2a+(n-1\left)d\right]$

$=\frac{8}{2}\left[2\right(-1)+(8-1\left)\right(-3\left)\right]$

$=4[-2-21]$

$=-92$