1.

The sum of the first 20 odd natural numbers is (a) 100 (b) 210 (c) 400 (d) 420

Answer»

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



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