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Application of Derivatives question

2014 · Shift 0 · Q33
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Application of Derivatives question

2014 · Shift 0 · Q33

JEE MainMathematicsApplication of DerivativesMCQ+4 / −1
If x=−1x=-1x=−1 and x=2x=2x=2 are extreme points of f(x)=α log⁡∣x∣+βx2+xf\left( x \right) = \alpha \,\log \left| x \right|+\beta {x^2} + xf(x)=αlog∣x∣+βx2+x then
  1. A
    α=2,β=−12\alpha = 2,\beta = - {1 \over 2}α=2,β=−21​
  2. B
    α=2,β=12\alpha = 2,\beta = {1 \over 2}α=2,β=21​
  3. C
    α=−6,β=12\alpha = - 6,\beta = {1 \over 2}α=−6,β=21​
  4. D
    α=−6,β=−12\alpha = - 6,\beta = -{1 \over 2}α=−6,β=−21​
View written solutionFree

Correct answer: A

  1. We are given f(x)=αlog⁡∣x∣+βx2+xf(x)=\alpha \log|x|+\beta x^2+xf(x)=αlog∣x∣+βx2+x and told that x=−1x=-1x=−1 and x=2x=2x=2 are extreme points.

  2. For an extreme point, we must have f′(x)=0.f'(x)=0.f′(x)=0.

First, differentiate: ddx(log⁡∣x∣)=1x(x≠0).\frac{d}{dx}(\log|x|)=\frac{1}{x}\quad (x\ne 0).dxd​(log∣x∣)=x1​(x=0). So, f′(x)=αx+2βx+1.f'(x)=\frac{\alpha}{x}+2\beta x+1.f′(x)=xα​+2βx+1.

  1. Since x=−1x=-1x=−1 is an extreme point, f′(−1)=0.f'(-1)=0.f′(−1)=0. Thus, α−1+2β(−1)+1=0\frac{\alpha}{-1}+2\beta(-1)+1=0−1α​+2β(−1)+1=0 −α−2β+1=0-\alpha-2\beta+1=0−α−2β+1=0 α+2β=1...(1)\alpha+2\beta=1 \quad ...(1)α+2β=1...(1)

  2. Since x=2x=2x=2 is an extreme point, f′(2)=0.f'(2)=0.f′(2)=0. Thus, α2+2β(2)+1=0\frac{\alpha}{2}+2\beta(2)+1=02α​+2β(2)+1=0 α2+4β+1=0...(2)\frac{\alpha}{2}+4\beta+1=0 \quad ...(2)2α​+4β+1=0...(2)

  3. Solve the system: From (1), α=1−2β.\alpha=1-2\beta.α=1−2β. Substitute into (2): 1−2β2+4β+1=0\frac{1-2\beta}{2}+4\beta+1=021−2β​+4β+1=0 12−β+4β+1=0\frac{1}{2}-\beta+4\beta+1=021​−β+4β+1=0 32+3β=0\frac{3}{2}+3\beta=023​+3β=0 3β=−323\beta=-\frac{3}{2}3β=−23​ β=−12.\beta=-\frac{1}{2}.β=−21​.

Now from (1), α+2(−12)=1\alpha+2\left(-\frac{1}{2}\right)=1α+2(−21​)=1 α−1=1\alpha-1=1α−1=1 α=2.\alpha=2.α=2.

  1. Hence, α=2,β=−12.\alpha=2,\quad \beta=-\frac{1}{2}.α=2,β=−21​. So the correct option is A.

  2. Verification of extreme points: f′′(x)=−αx2+2β.f''(x)=-\frac{\alpha}{x^2}+2\beta.f′′(x)=−x2α​+2β. Substitute α=2,β=−12\alpha=2,\beta=-\frac12α=2,β=−21​: f′′(x)=−2x2−1<0(x≠0).f''(x)=-\frac{2}{x^2}-1<0 \quad (x\ne 0).f′′(x)=−x22​−1<0(x=0). Therefore both x=−1x=-1x=−1 and x=2x=2x=2 are indeed points of local maximum, so they are extreme points.

  3. Comparison with stored answer: Stored correct answer = A, which matches our result.

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