# Mark the Correct Alternative in of the Following: If Y = 1 + X 1 ! + X 2 2 ! + X 3 3 ! + . . . Then D Y D X = - Mathematics

MCQ

Mark the correct alternative in of the following:

If$y = 1 + \frac{x}{1!} + \frac{x^2}{2!} + \frac{x^3}{3!} + . . .$then $\frac{dy}{dx} =$

#### Options

•  y + 1

• y − 1

• y

•  y2

#### Solution

$y = 1 + \frac{x}{1!} + \frac{x^2}{2!} + \frac{x^3}{3!} + . . .$

Differentiating both sides with respect to x, we get $\frac{dy}{dx} = \frac{d}{dx}\left( 1 + \frac{x}{1!} + \frac{x^2}{2!} + \frac{x^3}{3!} + . . . \right)$
$= \frac{d}{dx}\left( 1 \right) + \frac{d}{dx}\left( \frac{x}{1!} \right) + \frac{d}{dx}\left( \frac{x^2}{2!} \right) + \frac{d}{dx}\left( \frac{x^3}{3!} \right) + \frac{d}{dx}\left( \frac{x^4}{4!} \right) + . . .$
$= \frac{d}{dx}\left( 1 \right) + \frac{1}{1!}\frac{d}{dx}\left( x \right) + \frac{1}{2!}\frac{d}{dx}\left( x^2 \right) + \frac{1}{3!}\frac{d}{dx}\left( x^3 \right) + \frac{1}{4!}\frac{d}{dx}\left( x^4 \right) + . . .$
$= 0 + \frac{1}{1!} \times 1 + \frac{1}{2!} \times 2x + \frac{1}{3!} \times 3 x^2 + \frac{1}{4!} \times 4 x^3 + . . . \left( y = x^n \Rightarrow \frac{dy}{dx} = n x^{n - 1} \right)$

$= 1 + \frac{x}{1!} + \frac{x^2}{2!} + \frac{x^3}{3!} + . . . \left[ \frac{n}{n!} = \frac{1}{\left( n - 1 \right)!} \right]$
$= y$

$\therefore \frac{dy}{dx} = y$

Hence, the correct answer is option (c).

Concept: The Concept of Derivative - Algebra of Derivative of Functions
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#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 30 Derivatives
Q 3 | Page 47