Find the sum of first n natural numbers.

Answer:

First term, a = a₁ = 1

Common difference, d = a₂ – a₁ = 2 – 1 = 1

l = n

Sum of n terms = S

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

S = n/2 [2(1) + (n-1) 1]

S = n/2 [2 + n – 1]

S = n/2 [n – 1]

∴ The sum of the first n natural numbers is n(n-1)/2