Question

Find $S_{15}$ for an A.P $a=10,d=13$

Answer 1

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

$S_{15}=\frac{15}{2}(2\times 10+(15-1)13)$

$=\frac{15}{2}(20+182‬)$

$=\frac{15}{2}\times 202‬=1,515$

$\therefore S_{15}=1,515$