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Application of Derivatives question

2013 · Shift 0 · Q43
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Application of Derivatives question

2013 · Shift 0 · Q43

JEE MainMathematicsApplication of DerivativesMCQ+4 / −1
The real number kkk for which the equation, 2x3+3x+k=02{x^3} + 3x + k = 02x3+3x+k=0 has two distinct real roots in [0, 1]\left[ {0,\,1} \right][0,1]
  1. A
    lies between 1 and 2
  2. B
    lies between 2 and 3
  3. C
    lies between −1- 1−1 and 0
  4. D
    does not exist.
View written solutionFree

Correct answer: D

  1. Let f(x)=2x3+3x+k.f(x)=2x^3+3x+k.f(x)=2x3+3x+k. We want the equation 2x3+3x+k=02x^3+3x+k=02x3+3x+k=0 to have two distinct real roots in [0,1][0,1][0,1].

  2. Check the monotonicity of f(x)f(x)f(x). Differentiate: f′(x)=6x2+3=3(2x2+1).f'(x)=6x^2+3=3(2x^2+1).f′(x)=6x2+3=3(2x2+1). Since 6x2+3>0for all real x,6x^2+3>0 \quad \text{for all real } x,6x2+3>0for all real x, f(x)f(x)f(x) is strictly increasing on all of R\mathbb{R}R.

  3. A strictly increasing function can intersect the xxx-axis at most one point. That means the equation 2x3+3x+k=02x^3+3x+k=02x3+3x+k=0 cannot have two distinct real roots anywhere on the real line, and hence certainly not in [0,1][0,1][0,1].

  4. Therefore, there is no real number kkk for which the equation has two distinct real roots in [0,1][0,1][0,1].

  5. Hence the correct option is: D: does not exist\boxed{\text{D: does not exist}}D: does not exist​

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