Question

Find $S_{15}$ of an A.P: $4,8,12,16,......$

Answer 1

$a=4$,
$d=t_2=t_1=8-4=4$, where $t_2,t_1$ are the 1st and 2nd term of the A.P.
$S_{15}=\frac{n}{2}[2a+(n-1\left)d\right]$
$=\frac{15}{2}[2\times 4+(15-1)\times 4]$
$=\frac{15}{2}\times [8+56]$
$=\frac{15}{2}\times 64$
$=480$