Question

Find the sum of all even natural numbers from $2$ to $40$.

Answer 1

Sum of even natural numbers from $2$ to $40$ series is
$2+4+6+.....+40$
$=2(1+2+3+....+20)$
$=2\left[{\sum }_{i=1}^{n=20}i\right]$

$=2\left[\frac{20(20+1)}{2}\right][\sum n=\frac{n(n+1)}{2}]$
$=20\times 21$
$=420$