Question:medium

\(1 + \frac{2}{2!} + \frac{3}{3!} + \frac{4}{4!} + \cdots \infty\) equals

Show Hint

\(\sum_{n=1}^\infty \frac{n}{n!} = e\), \(\sum_{n=1}^\infty \frac{n^2}{n!} = 2e\).
Updated On: May 24, 2026
  • \(5e\)
  • \(4e\)
  • \(3e\)
  • \(2e\)
Show Solution

The Correct Option is D

Solution and Explanation

To solve this problem, we need to evaluate the infinite series: \(S = 1 + \frac{2}{2!} + \frac{3}{3!} + \frac{4}{4!} + \cdots \infty\). We will use the properties of exponential functions and series expansion to find the sum.

The function \(e^x\) is given by the series expansion: \(e^x = 1 + \frac{x}{1!} + \frac{x^2}{2!} + \frac{x^3}{3!} + \cdots\).

By differentiating the series expansion of \(e^x\) with respect to \(x\), we get: \(\frac{d}{dx}(e^x) = e^x = \frac{1}{0!} + \frac{x}{1!} + \frac{x^2}{2!} + \cdots\).

Now, multiply by \(x\)\(x \cdot e^x = x + \frac{x^2}{1!} + \frac{x^3}{2!} + \cdots\).

Differentiating again with respect to \(x\) gives: \((x \cdot e^x)' = e^x + x \cdot e^x = 1 + \frac{2x}{1!} + \frac{3x^2}{2!} + \cdots\).

Substitute \(x = 1\)\((x \cdot e^x)' \big|_{x=1} = e + e \cdot 1 = 2e\).

This result matches exactly with our series: \(1 + \frac{2}{2!} + \frac{3}{3!} + \frac{4}{4!} + \cdots = 2e\).

Therefore, the infinite series evaluates to \(2e\), which is the correct answer.

Thus, the correct option is:

\(<2e>\)

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