1.

In an A.P. 1 term is 1st and the last term is 20. The sum of all terms is 399, then n = …. (A) 42 (B) 38 (C) 21 (D) 19

Answer»

The correct answer is : (B) 38

Hints : 

S= n/2 (first terms + last terms)

∴ 399 = n/2 (1+20)

∴ 399 x 2 = 21n

∴ n = 798/21 = 38



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