Question

Find the sum of first $123$ even natural numbers.

Answer 1

We know that even natural numbers are $2,4,6,8,\mathrm{10.......}$

It is in $AP$ series

So,

$a=2$

$d=2$

$n=123$

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

$S=\frac{123}{2}[2\times 2+(123-1)\times 2]$

$S=\frac{123}{2}[4+122\times 2]$

$S=123\times 124$

$S=15252$