Write the sum of first n even natural numbers.

We need to find the sum of 2, 4, 6, 8...upto n terms.

Here, a = 2, d = 2

We know:

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

$=\frac{n}{2}\{2 \times 2+(n-1) 2\}$

$=n(n+1)$

Therefore, the sum of the first $n$ odd numbers is $n(n+1)$.