Question

In an A.P., if a_n = 1, a = 20 and S_n = 399, then n = ………………

A) 19 

B) 42 

C) 28 

D) 38

Answer 1

Correct option is (D) 38

We have a = 1, \(a_n = 20\) and \(S_n = 399\)

\(\because S_n=\frac n2(a+a_n)\)

\(\Rightarrow399=\frac n2(1+20)\)

\(\Rightarrow n=\frac{399\times2}{21}\)

\(=19\times2=38\)

Answer 2

Correct option is D) 38