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JEE Main 201816 AprilOnline
Exam
JEE Main
Year
2018
Date
16 April
Mode
Online
Subject
Mathematics
Chapter
Continuity and Differentiability

JEE Main 2018 · 16 April · Online · Mathematics · Continuity and Differentiability

If the function f defined as f(x) = (1)/(x) - k - 1 e ^ 2x - 1 , x...

If the function ff defined as f(x)=1x−k−1e2x−1,x≠0f\left(x\right) = \dfrac{1}{x} - \dfrac{k - 1}{{e}^{2x} - 1}, x \ne 0 is continuous at x=0x = 0, then ordered pair \left(k, f\left(0\right)\right) is equal to

Options

  1. A(2,1)\left(2, 1\right)
  2. B(3,1)\left(3, 1\right)
  3. C(3,2)\left(3, 2\right)
  4. D(13,2)\left(\dfrac{1}{3}, 2\right)

Correct answer

B. (3,1)\left(3, 1\right)

Step-by-step solution

Given f(x)=1x−k−1e2x−1,x≠0f\left(x\right) = \dfrac{1}{x} - \dfrac{k - 1}{{e}^{2x} - 1}, x \ne 0

and f(x)f\left(x\right) is continuous at x=0x = 0.

⇒f(0)=lim⁡x→0(1x−k−1e2x−1)⇒ f\left(0\right) = \lim_{x \to 0}\left(\dfrac{1}{x} - \dfrac{k - 1}{{e}^{2x} - 1}\right)

=lim⁡x→0(1+(2x)+12!(2x)2+…(−1−x(k−1)))2x2(e2x−12x)= \lim_{x \to 0}\dfrac{\left(1 + \left(2x\right) + \dfrac{1}{2 !}{\left(2x\right)}^{2} + \ldots \left(- 1 - x\left(k - 1\right)\right)\right)}{2{x}^{2}\left(\dfrac{{e}^{2x} - 1}{2x}\right)}

=lim⁡x→0(1+(2x)+12!(2x)2+…(−1−x(k−1)))2x2= \lim_{x \to 0}\dfrac{\left(1 + \left(2x\right) + \dfrac{1}{2 !}{\left(2x\right)}^{2} + \ldots \left(- 1 - x\left(k - 1\right)\right)\right)}{2{x}^{2}}

Clearly, k=3k = 3 and f(0)=1f\left(0\right) = 1. (( ∵∵Equating coefficient of x=0x = 0))

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