Question

The sum of the first 20 odd natural numbers is 

(a) 100 

(b) 210 

(c) 400 

(d) 420

Answer 1

Correct answer is (c) 400

The first 20 odd natural numbers are 1, 3, 5,.., 39.

These numbers are in AP.

Here. a = 1, l = 39 and n = 20

∴ Sum of first 20 odd natural numbers

= \(\frac{20}{2}(1+39)\)      [\(S_n=\frac{n}{2}(a+l)\)]

= 10 x 40

= 400