Join the Most Relevant JEE Main 2025 Test Series & get 99+ percentile! Join Now
Search any question & find its solution
Question: Answered & Verified by Expert
$\sum_{n=1}^{\infty} \frac{2 n^2+n+1}{n !}$ is equal to
MathematicsSequences and SeriesTS EAMCETTS EAMCET 2005
Options:
  • A $2 e-1$
  • B $2 e+1$
  • C $6 e-1$
  • D $6 e+1$
Solution:
1439 Upvotes Verified Answer
The correct answer is: $6 e-1$
$\begin{aligned} & \text { Let } S=\sum_{n=1}^{-} \frac{2 n^2+n+1}{n !} \\ & =\bar{\sum}_{n=1}\left(\frac{2 n}{(n-1) !}+\frac{1}{(n-1) !}+\frac{1}{n !}\right) \\ & =\bar{\sum}_{n=1}\left(\frac{2}{(n-2) !}+\frac{3}{(n-1) !}+\frac{1}{n !}\right) \\ & =2\left(1+\frac{1}{1 !}+\frac{1}{2 !}+\frac{1}{3 !}+\ldots \infty\right) \\ & +3\left(1+\frac{1}{1 !}+\frac{1}{2 !}+\ldots \infty\right)+\left(\frac{1}{1 !}+\frac{1}{2 !}+\frac{1}{3 !}+\ldots\right) \\ & =2 e+3 e+e-1 \\ & =6 e-1 \\ & \end{aligned}$

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.