Question

The sum of first 20 odd natural numbers is ______.

Options

• 100

• 210

• 400

• 420

Answer 1

The sum of first 20 odd natural numbers is 400.

Explanation:

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