Question

The first and last terms of an A.P. are 1 and 11. If the sum of its terms is 36, then the number of terms will be 

A. 5 

B. 6 

C. 7

D. 8

Answer 1

Correct answer  is (B). 6

a₁ = 1 (first term) 

a₁₁ = 11 = l (last term) 

S_n= 36 

But, S_n =\(\frac{n}{2}\) (a + l) 

36= \(\frac{n}{2}\)(12) 

n= 6