Medium MCQ +4 / -1 PYQ · JEE Mains 2025

The value of $\lim \limits_{n \rightarrow \infty}\left(\sum\limits_{k=1}^n \frac{k^3+6 k^2+11 k+5}{(k+3)!}\right)$ is :

  1. A <p>5/3</p> Correct answer
  2. B <p>2</p>
  3. C <p>4/3</p>
  4. D <p>7/3</p>

Solution

<p>$$\begin{aligned} & \lim _{n \rightarrow \infty} \sum_{k=1}^n \frac{k^3+6 k^2+11 k+5}{(k+3)!} \\ & =\lim _{n \rightarrow \infty} \sum_{k=1}^n \frac{k^3+6 k^2+11 k+6-1}{(k+3)!} \\ & =\lim _{n \rightarrow \infty} \sum_{k=1}^n \frac{(k+1)(k+2)(k+3)-1}{(k+3)!} \\ & =\lim _{n \rightarrow \infty} \sum_{k=1}^n \frac{(k+1)(k+2)(k+3)}{(k+3)!}-\frac{1}{(k+3)!} \\ & =\lim _{n \rightarrow \infty} \sum_{k=1}^n\left(\frac{1}{k!}-\frac{1}{(k+3)!}\right) \end{aligned}$$</p> <p>$$\begin{aligned} & =\lim _{\mathrm{n} \rightarrow \infty}\left(\frac{1}{1!}+\frac{1}{2!}+\frac{1}{3!}+\frac{1}{4!} \ldots+\frac{1}{\mathrm{n!}}-\frac{1}{4!}-\frac{1}{5!}-\frac{1}{6!} \ldots-\frac{1}{(\mathrm{n}+3)!}\right) \\ & \quad=\frac{1}{1}+\frac{1}{2}+\frac{1}{6}=\frac{10}{6}=\frac{5}{3} \end{aligned}$$</p>

About this question

Subject: Mathematics · Chapter: Limits, Continuity and Differentiability · Topic: Limits and Standard Results

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