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Limits Continuity and Differentiability question

2018 · 16 Apr · Shift 1 · Q29
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  5. /2018 · 16 Apr · Shift 1 · Q29

Limits Continuity and Differentiability question

2018 · 16 Apr · Shift 1 · Q29

JEE MainMathematicsLimits Continuity and DifferentiabilityMCQ+4 / −1
If the function f defined as f(x)=1x−k−1e2x−1,xe0,f\left( x \right) = {1 \over x} - {{k - 1} \over {{e^{2x}} - 1}},x e 0,f(x)=x1​−e2x−1k−1​,xe0, is continuous at x = 0, then the ordered pair (k, f(0)) is equal to :
  1. A
    (3, 2)
  2. B
    (3, 1)
  3. C
    (2, 1)
  4. D
    (13, 2)\left( {{1 \over 3},\,2} \right)(31​,2)
View written solutionFree

Correct answer: B

  1. For continuity at x=0x=0x=0, we must have f(0)=lim⁡x→0(1x−k−1e2x−1).f(0)=\lim_{x\to 0}\left(\frac{1}{x}-\frac{k-1}{e^{2x}-1}\right).f(0)=limx→0​(x1​−e2x−1k−1​). So first we evaluate this limit and ensure it is finite.

  2. Use the expansion of e2xe^{2x}e2x near x=0x=0x=0: e2x=1+2x+2x2+⋯e^{2x}=1+2x+2x^2+\cdotse2x=1+2x+2x2+⋯ Hence, e2x−1=2x+2x2+⋯=2x(1+x+⋯ ).e^{2x}-1=2x+2x^2+\cdots = 2x(1+x+\cdots).e2x−1=2x+2x2+⋯=2x(1+x+⋯). Therefore, 1e2x−1∼12x(x→0).\frac{1}{e^{2x}-1}\sim \frac{1}{2x}\quad (x\to 0).e2x−11​∼2x1​(x→0). So k−1e2x−1∼k−12x.\frac{k-1}{e^{2x}-1}\sim \frac{k-1}{2x}.e2x−1k−1​∼2xk−1​.

  3. Then near x=0x=0x=0, f(x)=1x−k−1e2x−1∼1x−k−12x.f(x)=\frac{1}{x}-\frac{k-1}{e^{2x}-1}\sim \frac{1}{x}-\frac{k-1}{2x}.f(x)=x1​−e2x−1k−1​∼x1​−2xk−1​. For the limit to exist finitely, the coefficient of 1x\frac{1}{x}x1​ must vanish: 1−k−12=0.1-\frac{k-1}{2}=0.1−2k−1​=0. Solve: 2−(k−1)=0⇒3−k=0⇒k=3.2-(k-1)=0 \Rightarrow 3-k=0 \Rightarrow k=3.2−(k−1)=0⇒3−k=0⇒k=3.

  4. Now substitute k=3k=3k=3: f(x)=1x−2e2x−1.f(x)=\frac{1}{x}-\frac{2}{e^{2x}-1}.f(x)=x1​−e2x−12​. We need f(0)=lim⁡x→0(1x−2e2x−1).f(0)=\lim_{x\to 0}\left(\frac{1}{x}-\frac{2}{e^{2x}-1}\right).f(0)=limx→0​(x1​−e2x−12​). Rewrite as f(x)=e2x−1−2xx(e2x−1).f(x)=\frac{e^{2x}-1-2x}{x(e^{2x}-1)}.f(x)=x(e2x−1)e2x−1−2x​.

  5. Use expansion again: e2x−1=2x+2x2+43x3+⋯e^{2x}-1=2x+2x^2+\frac{4}{3}x^3+\cdotse2x−1=2x+2x2+34​x3+⋯ So e2x−1−2x=2x2+43x3+⋯e^{2x}-1-2x=2x^2+\frac{4}{3}x^3+\cdotse2x−1−2x=2x2+34​x3+⋯ Also, x(e2x−1)=x(2x+2x2+⋯ )=2x2+2x3+⋯x(e^{2x}-1)=x\left(2x+2x^2+\cdots\right)=2x^2+2x^3+\cdotsx(e2x−1)=x(2x+2x2+⋯)=2x2+2x3+⋯ Thus, lim⁡x→0e2x−1−2xx(e2x−1)=22=1.\lim_{x\to 0}\frac{e^{2x}-1-2x}{x(e^{2x}-1)}=\frac{2}{2}=1.limx→0​x(e2x−1)e2x−1−2x​=22​=1. Therefore, f(0)=1.f(0)=1.f(0)=1.

  6. Hence the ordered pair is (k,f(0))=(3,1).(k,f(0))=(3,1).(k,f(0))=(3,1).

  7. Checking options:

  • A: (3,2)(3,2)(3,2) ✗
  • B: (3,1)(3,1)(3,1) ✓
  • C: (2,1)(2,1)(2,1) ✗
  • D: (13,2)\left(\frac13,2\right)(31​,2) ✗
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