Question

The n^th term of an arithmetic progression is -11 while sum of the first n terms is 66, when its first term is 22 then the value of n is 

A) 12 

B) 13 

C) 14 

D) 15

Answer 1

Correct option is (A) 12

We have \(a_n=-11,S_n=66\;\&\;a=22\)

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

\(\Rightarrow66=\frac n2(22-11)\)

\(\Rightarrow n=\frac{66\times2}{22-11}=\frac{66\times2}{11}\)

\(=6\times2=12\)

Answer 2

Correct option is A) 12