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Application of Derivatives question

2006 · Shift 0 · Q57
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Application of Derivatives question

2006 · Shift 0 · Q57

JEE MainMathematicsApplication of DerivativesMCQ+4 / −1
The function f(x)=x2+2xf\left( x \right) = {x \over 2} + {2 \over x}f(x)=2x​+x2​ has a local minimum at
  1. A
    x=2x=2x=2
  2. B
    x=−2x=-2x=−2
  3. C
    x=0x=0x=0
  4. D
    x=1x=1x=1
View written solutionFree

Correct answer: A

  1. Given function

    f(x)=x2+2x,x≠0f(x)=\frac{x}{2}+\frac{2}{x}, \qquad x\neq 0f(x)=2x​+x2​,x=0

    We need to find where this function has a local minimum.

  2. Find the first derivative

    f′(x)=ddx(x2+2x)=12−2x2f'(x)=\frac{d}{dx}\left(\frac{x}{2}+\frac{2}{x}\right)=\frac{1}{2}-\frac{2}{x^2}f′(x)=dxd​(2x​+x2​)=21​−x22​

  3. Find critical points

    Set f′(x)=0f'(x)=0f′(x)=0:

    12−2x2=0\frac{1}{2}-\frac{2}{x^2}=021​−x22​=0

    12=2x2\frac{1}{2}=\frac{2}{x^2}21​=x22​

    x2=4x^2=4x2=4

    x=±2x=\pm 2x=±2

  4. Use the second derivative test

    f′′(x)=ddx(12−2x2)=4x3f''(x)=\frac{d}{dx}\left(\frac{1}{2}-\frac{2}{x^2}\right)=\frac{4}{x^3}f′′(x)=dxd​(21​−x22​)=x34​

    • At x=2x=2x=2: f′′(2)=48=12>0f''(2)=\frac{4}{8}=\frac{1}{2}>0f′′(2)=84​=21​>0 So, x=2x=2x=2 is a local minimum.

    • At x=−2x=-2x=−2: f′′(−2)=4−8=−12<0f''(-2)=\frac{4}{-8}=-\frac{1}{2}<0f′′(−2)=−84​=−21​<0 So, x=−2x=-2x=−2 is a local maximum.

  5. Check the options

    • A: x=2x=2x=2 ✅ local minimum
    • B: x=−2x=-2x=−2 ❌ local maximum
    • C: x=0x=0x=0 ❌ not in domain
    • D: x=1x=1x=1 ❌ not a critical point

Therefore, the function has a local minimum at:

x=2\boxed{x=2}x=2​

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