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
The nth 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
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2 Answers
Correct option is (A) 12
We have \(a_n=-11,S_n=66\;\&\;a=22\)
\(\because S_n=\frac n2(a+a_n)\)
\(\Rightarrow66=\frac n2(22-11)\)
\(\Rightarrow n=\frac{66\times2}{22-11}=\frac{66\times2}{11}\)
\(=6\times2=12\)
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