Question

The sum of first 20 odd natural numbers is:

विकल्प

• A. 100

• B. 210

• C. 400

• D. 420

Answer 1

Odd natural numbers are in the pattern 1, 3, 5, 7, 9…,.

These numbers form an A.P. where a = 1, d = 3 − 1 = 2

We know that, `S_n=n/2[2a+(n-1)d]`

`thereforeS_20=20/2[2xx1+(20-1)xx2]`

`=20/2[2+19xx2]`

`=10[2+38]`

`=10xx40`

`=400`

Thus, the sum of first 20 odd natural numbers is 400.

Hence, the correct answer is C.