Question

In an AP, if a = 1, a_n = 20 and S_n = 399, then n is equal to 

A. 19  B. 21 C. 38 D. 42

Answer 1

C. 38

Given

First term, a = 1,

Nth term, a_n = 20,

Some of n terms, S_n = 299

Using the formula

Where S_n = Sum of first n terms

n = no of terms

l = a_n = last term

2S_n = n(a + a_n)

2(399) = n(1 + 20)

n = 798/21

n = 38