First n natural numbers are 1, 2, 3,…, n.

To find: sum of these n natural numbers.

The natural numbers forms an AP with first term 1 and common difference 1.

Now, Sum of first n terms of an AP is given by:

S_n = [2a + (n - 1)d]

where a is the first term and d is the common difference.

∴ Sum of first n natural numbers is given by:

S_n = [2(1) + (n - 1)(1)]

= [2 + n - 1]

= [n + 1]

∴ Sum of first n natural numbers is n(n + 1)/2.