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JEE Advanced 2021 Paper 1 · Mathematics · Differentiation and Application of Derivatives · medium

Let f(x)=x^2sin 1/x for x≠0 and f(0)=0.

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Let f(x)=x2sin⁡1xf(x)=x^{2}\sin\dfrac1x for x≠0x\ne0 and f(0)=0f(0)=0. Which of the following statements is (are) correct?
  1. A. ff is differentiable at x=0x=0 and f′(0)=0f'(0)=0
  2. B. f′f' is continuous at x=0x=0
  3. C. f′f' is bounded on the interval (−1,1)∖{0}(-1,1)\setminus\{0\}
  4. D. f′′(0)f''(0) exists
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Answer: A, C

(A) true: f(h)−f(0)h=hsin⁡1h\dfrac{f(h)-f(0)}{h}=h\sin\dfrac1h and ∣hsin⁡(1/h)∣≤∣h∣→0|h\sin(1/h)|\le|h|\to0, so f′(0)=0f'(0)=0. For x≠0x\ne0, f′(x)=2xsin⁡1x−cos⁡1xf'(x)=2x\sin\dfrac1x-\cos\dfrac1x. (B) false: as x→0x\to0 the first term tends to 00 but cos⁡(1/x)\cos(1/x) oscillates between −1-1 and 11 without a limit, so f′(x)↛f′(0)=0f'(x)\not\to f'(0)=0. (C) true: ∣f′(x)∣≤2∣x∣+1<3|f'(x)|\le2|x|+1<3 on (−1,1)∖{0}(-1,1)\setminus\{0\}, and f′(0)=0f'(0)=0 as well. (D) false: f′(h)−f′(0)h=2sin⁡1h−cos⁡(1/h)h\dfrac{f'(h)-f'(0)}{h}=2\sin\dfrac1h-\dfrac{\cos(1/h)}{h} is unbounded and has no limit as h→0h\to0. Thus ff is differentiable everywhere with a bounded but discontinuous derivative, so exactly (A) and (C) hold.
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