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JEE Advanced 2016 Paper 1 · Mathematics · Differentiation and Application of Derivatives · hard

Let f(x)=∫0^x (t-1)(t-3)(t-2)^2 dt.

One or more options correct

Let f(x)=∫0x(t−1)(t−3)(t−2)2 dtf(x)=\displaystyle\int_0^x (t-1)(t-3)(t-2)^2\,dt. Which of the following is/are correct?
  1. A. ff has a local maximum at x=1x=1, a local minimum at x=3x=3, and no other local extremum
  2. B. f(1)−f(3)=215f(1)-f(3)=\dfrac{2}{15}
  3. C. The graph of y=f(x)y=f(x) has exactly three points of inflection
  4. D. The equation f(x)=0f(x)=0 has three distinct real roots
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Answer: A, C

We have f′(x)=(x−1)(x−3)(x−2)2f'(x)=(x-1)(x-3)(x-2)^2. The sign is ++ for x<1x<1, −- on (1,3)(1,3) (the square factor does not change it at 22), and ++ for x>3x>3. So ff has a local maximum at 11, a local minimum at 33, and x=2x=2 is only a stationary point; (A) is true. Expanding, f(x)=x55−2x4+23x33−14x2+12xf(x)=\frac{x^5}{5}-2x^4+\frac{23x^3}{3}-14x^2+12x, giving f(1)=5815f(1)=\frac{58}{15} and f(3)=5415f(3)=\frac{54}{15}, so f(1)−f(3)=415f(1)-f(3)=\frac{4}{15} and (B) is false. Next f′′(x)=2(x−2)(2x2−8x+7)f''(x)=2(x-2)(2x^2-8x+7) has three simple roots 2, 2±122,\,2\pm\tfrac{1}{\sqrt2}, each a sign change, so there are three inflection points and (C) is true. Since f(0)=0f(0)=0, ff is increasing up to a positive local maximum and stays positive afterwards, and the quintic's other roots are complex, so f=0f=0 has only one real root and (D) is false.
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