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Limits Continuity and Differentiability question

2021 · 27 Jul · Shift 1 · Q34
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  5. /2021 · 27 Jul · Shift 1 · Q34

Limits Continuity and Differentiability question

2021 · 27 Jul · Shift 1 · Q34

JEE MainMathematicsLimits Continuity and DifferentiabilityMCQ+4 / −1
Let f : R →\to→ R be a function such that f(2) = 4 and f'(2) = 1. Then, the value of lim⁡x→2x2f(2)−4f(x)x−2\mathop {\lim }\limits_{x \to 2} {{{x^2}f(2) - 4f(x)} \over {x - 2}}x→2lim​x−2x2f(2)−4f(x)​ is equal to :
  1. A
    4
  2. B
    8
  3. C
    16
  4. D
    12
View written solutionFree

Correct answer: D

  1. We are given: f(2)=4,f′(2)=1f(2)=4, \qquad f'(2)=1f(2)=4,f′(2)=1

    We need to evaluate L=lim⁡x→2x2f(2)−4f(x)x−2.L=\lim_{x\to 2}\frac{x^2f(2)-4f(x)}{x-2}.L=limx→2​x−2x2f(2)−4f(x)​.

  2. Substitute f(2)=4f(2)=4f(2)=4:

    =4\lim_{x\to 2}\frac{x^2-f(x)}{x-2}.$$
  3. Since f(2)=4=22f(2)=4=2^2f(2)=4=22, the numerator becomes 000 at x=2x=2x=2, so we can rewrite: x2−f(x)=(x2−4)−(f(x)−4).x^2-f(x)=(x^2-4)-(f(x)-4).x2−f(x)=(x2−4)−(f(x)−4). Therefore, x2−f(x)x−2=x2−4x−2−f(x)−4x−2.\frac{x^2-f(x)}{x-2}=\frac{x^2-4}{x-2}-\frac{f(x)-4}{x-2}.x−2x2−f(x)​=x−2x2−4​−x−2f(x)−4​.

  4. Now take limits separately: lim⁡x→2x2−4x−2=lim⁡x→2(x+2)=4,\lim_{x\to 2}\frac{x^2-4}{x-2} = \lim_{x\to 2}(x+2)=4,limx→2​x−2x2−4​=limx→2​(x+2)=4, and lim⁡x→2f(x)−4x−2=lim⁡x→2f(x)−f(2)x−2=f′(2)=1.\lim_{x\to 2}\frac{f(x)-4}{x-2}=\lim_{x\to 2}\frac{f(x)-f(2)}{x-2}=f'(2)=1.limx→2​x−2f(x)−4​=limx→2​x−2f(x)−f(2)​=f′(2)=1.

  5. Hence, lim⁡x→2x2−f(x)x−2=4−1=3.\lim_{x\to 2}\frac{x^2-f(x)}{x-2}=4-1=3.limx→2​x−2x2−f(x)​=4−1=3.

  6. Multiply by 444: L=4⋅3=12.L=4\cdot 3=12.L=4⋅3=12.

  7. Therefore, the required value is 12.\boxed{12}.12​.

  8. Checking options:

    • A: 444 ❌
    • B: 888 ❌
    • C: 161616 ❌
    • D: 121212 ✅

So the correct option is D.

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