Sequence And Series Question 455
Question: The $ n^{th} $ term of an A.P. is $ 3n-1 $ .Choose from the following the sum of its first five terms
[MP PET 1983]
Options:
A) 14
B) 35
C) 80
D) 40
Show Answer
Answer:
Correct Answer: D
Solution:
Here $ T_{n}=3n-1 $ , putting $ n=1,\ 2,\ 3,\ 4,\ 5 $ we get first five terms, $ 2,\ 5,\ 8,\ 11,\ 14 $ Hence sum is $ n^{th} $ . Aliter: $ S_{n}=\Sigma T_{n}=3\Sigma n-\Sigma 1=\frac{3n(n+1)}{2}-n $
$ \Rightarrow $ $ S_5=\frac{3.5.6}{2}-5=40 $ .
BETA
