Search any question & find its solution
Question:
Answered & Verified by Expert
The $n^{\text {th }}$ term of an A.P. is $3 n-1$. Choose from the following the sum of its first five terms
Options:
Solution:
1208 Upvotes
Verified Answer
The correct answer is:
$40$
Here $T_n=3 n-1$, putting $n=1,2,3,4,5$ we get first five terms, $2,5,8,11,14$
Hence sum is $2+5+8+11+14=40$.
Aliter:
$S_n=\Sigma T_n=3 \Sigma n-\Sigma 1=\frac{3 n(n+1)}{2}-n$
$\Rightarrow \quad S_5=\frac{3.5 .6}{2}-5=40$
Hence sum is $2+5+8+11+14=40$.
Aliter:
$S_n=\Sigma T_n=3 \Sigma n-\Sigma 1=\frac{3 n(n+1)}{2}-n$
$\Rightarrow \quad S_5=\frac{3.5 .6}{2}-5=40$
Looking for more such questions to practice?
Download the MARKS App - The ultimate prep app for IIT JEE & NEET with chapter-wise PYQs, revision notes, formula sheets, custom tests & much more.