1.

The sum of first 20 odd natural numbers is A. 100 B. 210 C. 400 D. 420

Answer»

Here A.P is 1,3, 5, …… 

a= 1, d= 2 and n = 20 

Sn = n/2(2a + (n–1) d) 

Sn = 20/2 (2 x 1 + (19) 2) 

Sn = 10 (2 + 38) 

Sn= 400



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