Question

Find the sum of the following arithmetic progression $9,12,15,........$ up to $16$ terms.

Answer 1

Given,

$a=9$

$d=12-9=3$

$n=16$

Formula:

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

$=\frac{16}{2}\left[2\right(9)+(16-1\left)3\right]$

$=8[18+45]$

$=504$

Hence, this is the answer.