Question

Find the sum of the first $12$ natural numbers each of which is a multiple of $7$.

Answer 1

The first $12$ natural numbers which are multiples of $7$ are:

$7,14,21,28,35,42,49,56,63,70,77$ and $84$

This is an A.P where,

$a=7,d=7,l=84$ and $n=12$

So,

$S_{12}=12/2[7+84]$

$=6\left[91\right]$

$=546$