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Differentiation question

2008 · Shift 1 · Q31
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  5. /2008 · Shift 1 · Q31

Differentiation question

2008 · Shift 1 · Q31

JEE AdvancedMathematicsDifferentiationMCQ+3 / −1
Consider the functions defined implicitly by the equation y3−3y+x=0y^3-3y+x=0y3−3y+x=0 on various intervals in the real line. If x∈(−∞,−2)∪(2,∞)x\in(-\infty,-2)\cup(2,\infty)x∈(−∞,−2)∪(2,∞), the equation implicitly defines a unique real valued differentiable function y=f(x)y=f(x)y=f(x). If x∈(−2,2)x\in(-2,2)x∈(−2,2), the equation implicitly defines a unique real valued differentiable function y=g(x)y=g(x)y=g(x) satisfying g(0)=0g(0)=0g(0)=0If f(−102)=22,f\left( { - 10\sqrt 2 } \right) = 2\sqrt 2 ,f(−102​)=22​, then f′′(−102)=f''\left( { - 10\sqrt 2 } \right) =f′′(−102​)=
  1. A
    427332{{4\sqrt 2 } \over {{7^3}{3^2}}}733242​​
  2. B
    −427332-{{4\sqrt 2 } \over {{7^3}{3^2}}}−733242​​
  3. C
    42733{{4\sqrt 2 } \over {{7^3}3}}73342​​
  4. D
    −42733-{{4\sqrt 2 } \over {{7^3}3}}−73342​​
View written solutionFree

Correct answer: B

Step-by-step Solution:

  1. Understand the Implicit Equation and the Given Point The function y=f(x)y = f(x)y=f(x) is defined implicitly by the equation y3−3y+x=0y^3 - 3y + x = 0y3−3y+x=0. We are given a point on this function: when x=−102x = -10\sqrt{2}x=−102​, the corresponding yyy value is f(−102)=22f(-10\sqrt{2}) = 2\sqrt{2}f(−102​)=22​.

    First, let's verify that this point satisfies the equation: (22)3−3(22)+(−102)=(8×22)−62−102(2\sqrt{2})^3 - 3(2\sqrt{2}) + (-10\sqrt{2}) = (8 \times 2\sqrt{2}) - 6\sqrt{2} - 10\sqrt{2}(22​)3−3(22​)+(−102​)=(8×22​)−62​−102​ =162−62−102=162−162=0= 16\sqrt{2} - 6\sqrt{2} - 10\sqrt{2} = 16\sqrt{2} - 16\sqrt{2} = 0=162​−62​−102​=162​−162​=0 The point (−102,22)(-10\sqrt{2}, 2\sqrt{2})(−102​,22​) indeed lies on the curve.

  2. Find the First Derivative (f′(x)f'(x)f′(x)) We differentiate the given equation implicitly with respect to xxx: ddx(y3−3y+x)=ddx(0)\frac{d}{dx}(y^3 - 3y + x) = \frac{d}{dx}(0)dxd​(y3−3y+x)=dxd​(0) 3y2dydx−3dydx+1=03y^2 \frac{dy}{dx} - 3 \frac{dy}{dx} + 1 = 03y2dxdy​−3dxdy​+1=0 Factor out dydx\frac{dy}{dx}dxdy​: (3y2−3)dydx=−1(3y^2 - 3) \frac{dy}{dx} = -1(3y2−3)dxdy​=−1 Solve for dydx\frac{dy}{dx}dxdy​: f′(x)=dydx=−13y2−3f'(x) = \frac{dy}{dx} = \frac{-1}{3y^2 - 3}f′(x)=dxdy​=3y2−3−1​

  3. Find the Second Derivative (f′′(x)f''(x)f′′(x)) Now, we differentiate the expression for f′(x)f'(x)f′(x) with respect to xxx using the chain rule. It's easier to write f′(x)=−(3y2−3)−1f'(x) = -(3y^2 - 3)^{-1}f′(x)=−(3y2−3)−1. f′′(x)=d2ydx2=ddx[−(3y2−3)−1]f''(x) = \frac{d^2y}{dx^2} = \frac{d}{dx} \left[ -(3y^2 - 3)^{-1} \right]f′′(x)=dx2d2y​=dxd​[−(3y2−3)−1] f′′(x)=−(−1)(3y2−3)−2⋅ddx(3y2−3)f''(x) = -(-1)(3y^2 - 3)^{-2} \cdot \frac{d}{dx}(3y^2 - 3)f′′(x)=−(−1)(3y2−3)−2⋅dxd​(3y2−3) f′′(x)=(3y2−3)−2⋅(6ydydx)f''(x) = (3y^2 - 3)^{-2} \cdot (6y \frac{dy}{dx})f′′(x)=(3y2−3)−2⋅(6ydxdy​) f′′(x)=6y(3y2−3)2dydxf''(x) = \frac{6y}{(3y^2 - 3)^2} \frac{dy}{dx}f′′(x)=(3y2−3)26y​dxdy​

    Now, substitute the expression for dydx\frac{dy}{dx}dxdy​ we found in Step 2: f′′(x)=6y(3y2−3)2(−13y2−3)f''(x) = \frac{6y}{(3y^2 - 3)^2} \left( \frac{-1}{3y^2 - 3} \right)f′′(x)=(3y2−3)26y​(3y2−3−1​) f′′(x)=−6y(3y2−3)3f''(x) = \frac{-6y}{(3y^2 - 3)^3}f′′(x)=(3y2−3)3−6y​

  4. Evaluate the Second Derivative at the Given Point We need to find f′′(−102)f''(-10\sqrt{2})f′′(−102​). We use the corresponding value of y=22y = 2\sqrt{2}y=22​. Substitute y=22y = 2\sqrt{2}y=22​ into the expression for f′′(x)f''(x)f′′(x): f′′(−102)=−6(22)(3(22)2−3)3f''(-10\sqrt{2}) = \frac{-6(2\sqrt{2})}{(3(2\sqrt{2})^2 - 3)^3}f′′(−102​)=(3(22​)2−3)3−6(22​)​

    First, calculate the term in the denominator's parenthesis: 3(22)2−3=3(4×2)−3=3(8)−3=24−3=213(2\sqrt{2})^2 - 3 = 3(4 \times 2) - 3 = 3(8) - 3 = 24 - 3 = 213(22​)2−3=3(4×2)−3=3(8)−3=24−3=21

    Now substitute this back into the expression for f′′(−102)f''(-10\sqrt{2})f′′(−102​): f′′(−102)=−122(21)3f''(-10\sqrt{2}) = \frac{-12\sqrt{2}}{(21)^3}f′′(−102​)=(21)3−122​​

  5. Simplify the Result We can simplify the fraction. Note that 21=3×721 = 3 \times 721=3×7 and 12=3×412 = 3 \times 412=3×4. f′′(−102)=−122(3×7)3=−12233×73f''(-10\sqrt{2}) = \frac{-12\sqrt{2}}{(3 \times 7)^3} = \frac{-12\sqrt{2}}{3^3 \times 7^3}f′′(−102​)=(3×7)3−122​​=33×73−122​​ f′′(−102)=−(3×4)233×73=−4232×73f''(-10\sqrt{2}) = \frac{-(3 \times 4)\sqrt{2}}{3^3 \times 7^3} = \frac{-4\sqrt{2}}{3^2 \times 7^3}f′′(−102​)=33×73−(3×4)2​​=32×73−42​​

    This matches option B.

Conclusion

The second derivative at the given point is f′′(−102)=−4273⋅32f''(-10\sqrt{2}) = -\frac{4\sqrt{2}}{7^3 \cdot 3^2}f′′(−102​)=−73⋅3242​​. Comparing this with the options provided, the correct option is B.

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