Question

Find sum of first $22$ terms of AP $8,3,-2,........$ .

Answer 1

$a=8$

$d=3-8=-5$

$S_{22}=\frac{n}{2}[2a+(n-1\left)d\right]$

$S_{22}=\frac{22}{2}[2a+21d]$

$S_{22}=11\left[2\right(8)+21(-5\left)\right]$

$S_{22}=11(-89)$

$=-979$.