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JEE Advanced 2019 Paper 2 · Mathematics · Indefinite and Definite Integrals · hard

For real x define F(x)=∫-1^1|t-x| dt.

One or more options correct

For real xx define F(x)=∫−11∣t−x∣ dtF(x)=\displaystyle\int_{-1}^{1}|t-x|\,dt. Which of the following statements is (are) correct?
  1. A. F(x)=x2+1F(x)=x^2+1 for ∣x∣≤1|x|\le1 and F(x)=2∣x∣F(x)=2|x| for ∣x∣>1|x|>1.
  2. B. FF is differentiable at every real xx.
  3. C. F′′(1)F''(1) exists.
  4. D. ∫−22F(x) dx=263\displaystyle\int_{-2}^{2}F(x)\,dx=\dfrac{26}{3}.
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Answer: A, B, D

For ∣x∣≤1|x|\le1 split at t=xt=x: ∫−1x(x−t) dt+∫x1(t−x) dt=(x+1)22+(1−x)22=x2+1\int_{-1}^{x}(x-t)\,dt+\int_x^1(t-x)\,dt=\frac{(x+1)^2}2+\frac{(1-x)^2}2=x^2+1. For x>1x>1 every t∈[−1,1]t\in[-1,1] has t<xt<x, giving ∫−11(x−t) dt=2x\int_{-1}^{1}(x-t)\,dt=2x, and similarly 2∣x∣2|x| for x<−1x<-1: (A) correct. At x=1x=1 the left derivative of x2+1x^2+1 is 22 and the right derivative of 2x2x is 22, and the same holds at x=−1x=-1, so FF is differentiable everywhere (B correct). But F′′(1−)=2F''(1^-)=2 while F′′(1+)=0F''(1^+)=0, so F′′(1)F''(1) does not exist (C incorrect). Using evenness, ∫−22F=∫−11(x2+1) dx+2∫122x dx=83+6=263\int_{-2}^{2}F=\int_{-1}^{1}(x^2+1)\,dx+2\int_1^2 2x\,dx=\frac83+6=\frac{26}3 (D correct). Answer: A, B and D.
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