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JEE Advanced 2021 Paper 2 · Mathematics · Limits · medium

Define f(x)=lim n→∞((x+1) x^2n+x^2/(x^2n+1) for every real x for which the limit exists.

One or more options correct

Define f(x)=lim⁡n→∞(x+1) x2n+x2x2n+1f(x)=\displaystyle\lim_{n\to\infty}\frac{(x+1)\,x^{2n}+x^{2}}{x^{2n}+1} for every real xx for which the limit exists. Which of the following statements is/are correct?
  1. A. ff is discontinuous at exactly two points
  2. B. ∫−22f(x) dx=3\displaystyle\int_{-2}^{2}f(x)\,dx=3
  3. C. f(1)+f(−1)=3f(1)+f(-1)=3
  4. D. ff is differentiable at x=0x=0 with f′(0)=1f'(0)=1
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Answer: A

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+1f(x)=x+1; at x=1x=1 the expression is 2+12=32\frac{2+1}{2}=\frac32, and at x=−1x=-1 it is 0+12=12\frac{0+1}{2}=\frac12. At x=1x=1 the one-sided limits are 11 and 22, and at x=−1x=-1 they are 11 (from the right) and 00 (from the left), and neither matches the function value, so there are exactly two discontinuities and (A) is true. Splitting the integral, ∫−2−1(x+1)dx+∫−11x2dx+∫12(x+1)dx=−12+23+52=83≠3\int_{-2}^{-1}(x+1)dx+\int_{-1}^{1}x^2dx+\int_1^2(x+1)dx=-\frac12+\frac23+\frac52=\frac83\ne3, so (B) is false. f(1)+f(−1)=32+12=2f(1)+f(-1)=\frac32+\frac12=2, so (C) is false. Near 00, f(x)=x2f(x)=x^2, so f′(0)=0f'(0)=0, not 11, and (D) is false. A high-power numerical evaluation agrees.
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