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JEE Advanced 2016 Paper 1 · Mathematics · Limits · hard

Let f(x)=lim n→∞(x^2n(x+2)+x^2/(x^2n+1).

One or more options correct

Let f(x)=lim⁡n→∞x2n(x+2)+x2x2n+1f(x)=\displaystyle\lim_{n\to\infty}\frac{x^{2n}(x+2)+x^2}{x^{2n}+1}. Which of the following is/are correct?
  1. A. ff is continuous at x=−1x=-1
  2. B. ff is differentiable at x=−1x=-1
  3. C. ff has a jump discontinuity at x=1x=1
  4. D. ∫02f(x) dx=113\displaystyle\int_0^2 f(x)\,dx=\dfrac{11}{3}
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Answer: A, C

Evaluate the limit by cases. For ∣x∣<1|x|<1, x2n→0x^{2n}\to0 so f(x)=x2f(x)=x^2. For ∣x∣>1|x|>1, x2n→∞x^{2n}\to\infty so f(x)=x+2f(x)=x+2. At ∣x∣=1|x|=1, x2n=1x^{2n}=1 and f(x)=x+2+x22f(x)=\dfrac{x+2+x^2}{2}, giving f(1)=2f(1)=2 and f(−1)=1f(-1)=1. At x=−1x=-1: the left limit is −1+2=1-1+2=1, the right limit is 11, and f(−1)=1f(-1)=1, so ff is continuous and (A) is true. The left derivative there is 11 while the right derivative is 2x=−22x=-2, so (B) is false. At x=1x=1: left limit 11, right limit 33, value 22, a jump, so (C) is true. Finally ∫02f=∫01x2dx+∫12(x+2)dx=13+72=236\int_0^2f=\int_0^1x^2dx+\int_1^2(x+2)dx=\tfrac13+\tfrac72=\tfrac{23}{6}, not 113\tfrac{11}{3}, so (D) is false (numerical quadrature agrees).
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