Question

The sum of first 20 odd natural numbers is

(a) 100
(b) 210
(c) 400
(d) 420

Answer 1

Let a be the first term and d be the common difference.

We know that, sum of first n terms = S_n = $\frac{n}{2}$[2a + (n − 1)d]

The given series is 1 + 3 + 5 + ......

First term = a = 1.

Common difference = d = 3 − 1 = 2

∴ S₂₀ = $\frac{20}{2}$[2 × 1 + (20 − 1)2]
= 10(2 + 19 × 2)
= 10(40)
= 400

Hence, the correct option is (c).