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Limits Continuity and Differentiability question

2019 · 11 Jan · Shift 2 · Q30
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  5. /2019 · 11 Jan · Shift 2 · Q30

Limits Continuity and Differentiability question

2019 · 11 Jan · Shift 2 · Q30

JEE MainMathematicsLimits Continuity and DifferentiabilityMCQ+4 / −1
lim⁡x→0xcot⁡(4x)sin⁡2xcot⁡2(2x)\mathop {\lim }\limits_{x \to 0} {{x\cot \left( {4x} \right)} \over {{{\sin }^2}x{{\cot }^2}\left( {2x} \right)}}x→0lim​sin2xcot2(2x)xcot(4x)​ is equal to :
  1. A
    0
  2. B
    4
  3. C
    1
  4. D
    2
View written solutionFree

Correct answer: C

  1. We need to evaluate L=lim⁡x→0xcot⁡(4x)sin⁡2x cot⁡2(2x).L=\lim_{x\to 0}\frac{x\cot(4x)}{\sin^2 x\,\cot^2(2x)}.L=limx→0​sin2xcot2(2x)xcot(4x)​.

  2. Rewrite cotangent in terms of sine and cosine: cot⁡(4x)=cos⁡4xsin⁡4x,cot⁡(2x)=cos⁡2xsin⁡2x.\cot(4x)=\frac{\cos 4x}{\sin 4x}, \qquad \cot(2x)=\frac{\cos 2x}{\sin 2x}.cot(4x)=sin4xcos4x​,cot(2x)=sin2xcos2x​. So L=lim⁡x→0x cos⁡4xsin⁡4xsin⁡2x cos⁡22xsin⁡22x.L=\lim_{x\to 0}\frac{x\,\dfrac{\cos 4x}{\sin 4x}}{\sin^2 x\,\dfrac{\cos^2 2x}{\sin^2 2x}}.L=limx→0​sin2xsin22xcos22x​xsin4xcos4x​​.

  3. Simplify: L=lim⁡x→0x⋅cos⁡4xsin⁡4x⋅sin⁡22xsin⁡2xcos⁡22x.L=\lim_{x\to 0}x\cdot \frac{\cos 4x}{\sin 4x}\cdot \frac{\sin^2 2x}{\sin^2 x\cos^2 2x}.L=limx→0​x⋅sin4xcos4x​⋅sin2xcos22xsin22x​. Using sin⁡2x=2sin⁡xcos⁡x,\sin 2x=2\sin x\cos x,sin2x=2sinxcosx, we get sin⁡22x=4sin⁡2xcos⁡2x.\sin^2 2x=4\sin^2 x\cos^2 x.sin22x=4sin2xcos2x. Therefore, L=lim⁡x→0x⋅cos⁡4xsin⁡4x⋅4sin⁡2xcos⁡2xsin⁡2xcos⁡22xL=\lim_{x\to 0}x\cdot \frac{\cos 4x}{\sin 4x}\cdot \frac{4\sin^2 x\cos^2 x}{\sin^2 x\cos^2 2x}L=limx→0​x⋅sin4xcos4x​⋅sin2xcos22x4sin2xcos2x​ =lim⁡x→04x⋅cos⁡4xsin⁡4x⋅cos⁡2xcos⁡22x.=\lim_{x\to 0}4x\cdot \frac{\cos 4x}{\sin 4x}\cdot \frac{\cos^2 x}{\cos^2 2x}.=limx→0​4x⋅sin4xcos4x​⋅cos22xcos2x​.

  4. Now use the standard limit lim⁡t→0sin⁡tt=1⇒lim⁡t→0tsin⁡t=1.\lim_{t\to 0}\frac{\sin t}{t}=1 \quad \Rightarrow \quad \lim_{t\to 0}\frac{t}{\sin t}=1.limt→0​tsint​=1⇒limt→0​sintt​=1. Hence, lim⁡x→04xsin⁡4x=1.\lim_{x\to 0}\frac{4x}{\sin 4x}=1.limx→0​sin4x4x​=1. Also, cos⁡4x→1,cos⁡2x→1,cos⁡22x→1(x→0).\cos 4x\to 1, \qquad \cos^2 x\to 1, \qquad \cos^2 2x\to 1 \quad (x\to 0).cos4x→1,cos2x→1,cos22x→1(x→0). So, L=1⋅1⋅1=1.L=1\cdot 1\cdot 1=1.L=1⋅1⋅1=1.

  5. Therefore the value of the limit is 1.\boxed{1}.1​.

  6. Checking options:

  • A: 000 ❌
  • B: 444 ❌
  • C: 111 ✅
  • D: 222 ❌

Hence, the correct option is C.

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