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Definite Integration question

2008 · Shift 1 · Q29
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Definite Integration question

2008 · Shift 1 · Q29

JEE AdvancedMathematicsDefinite IntegrationMCQ+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)=0∫−11g′(x)dx=\int\limits_{ - 1}^1 {g'\left( x \right)dx = }−1∫1​g′(x)dx=
  1. A
    2g(−1)2g(-1)2g(−1)
  2. B
    000
  3. C
    −2g(1)-2g(1)−2g(1)
  4. D
    2g(1)2g(1)2g(1)
View written solutionFree

Correct answer: D

Step-by-step Solution:

  1. Understand the Goal: We are asked to evaluate the definite integral I=∫−11g′(x)dxI = \int\limits_{ - 1}^1 {g'\left( x \right)dx}I=−1∫1​g′(x)dx where y=g(x)y=g(x)y=g(x) is an implicitly defined function.

  2. Apply the Fundamental Theorem of Calculus: The Fundamental Theorem of Calculus states that if G′(x)=g(x)G'(x) = g(x)G′(x)=g(x), then ∫abg(x)dx=G(b)−G(a)\int_a^b g(x) dx = G(b) - G(a)∫ab​g(x)dx=G(b)−G(a). In our case, the integrand is g′(x)g'(x)g′(x), and its antiderivative is g(x)g(x)g(x). Therefore, we can directly evaluate the integral as: I=∫−11g′(x)dx=[g(x)]−11=g(1)−g(−1)I = \int\limits_{ - 1}^1 {g'\left( x \right)dx} = \left[ {g\left( x \right)} \right]_{ - 1}^1 = g\left( 1 \right) - g\left( { - 1} \right)I=−1∫1​g′(x)dx=[g(x)]−11​=g(1)−g(−1)

  3. Analyze the Properties of the function g(x)g(x)g(x): The function y=g(x)y = g(x)y=g(x) is defined by the equation y3−3y+x=0y^3 - 3y + x = 0y3−3y+x=0 for x∈(−2,2)x \in (-2, 2)x∈(−2,2) with the condition g(0)=0g(0) = 0g(0)=0. Let's check for symmetry in the defining equation. Let's replace (x,y)(x, y)(x,y) with (−x,−y)(-x, -y)(−x,−y) in the equation: (−y)3−3(−y)+(−x)=0(-y)^3 - 3(-y) + (-x) = 0(−y)3−3(−y)+(−x)=0 −y3+3y−x=0-y^3 + 3y - x = 0−y3+3y−x=0 −(y3−3y+x)=0 -(y^3 - 3y + x) = 0−(y3−3y+x)=0 This is the same as the original equation. This means that if a point (x,y)(x, y)(x,y) lies on the curve, then the point (−x,−y)(-x, -y)(−x,−y) also lies on the curve.

  4. Prove that g(x)g(x)g(x) is an odd function: We are given that for x∈(−2,2)x \in (-2, 2)x∈(−2,2), there is a unique differentiable function y=g(x)y = g(x)y=g(x) that satisfies y3−3y+x=0y^3 - 3y + x = 0y3−3y+x=0 and g(0)=0g(0) = 0g(0)=0. Let's define a new function h(x)=−g(−x)h(x) = -g(-x)h(x)=−g(−x). We want to check if h(x)h(x)h(x) satisfies the same conditions as g(x)g(x)g(x).

    • Check the implicit equation: Substitute y=h(x)y = h(x)y=h(x) into the equation: (h(x))3−3(h(x))+x=(−g(−x))3−3(−g(−x))+x(h(x))^3 - 3(h(x)) + x = (-g(-x))^3 - 3(-g(-x)) + x(h(x))3−3(h(x))+x=(−g(−x))3−3(−g(−x))+x =−[g(−x)]3+3g(−x)+x= -[g(-x)]^3 + 3g(-x) + x=−[g(−x)]3+3g(−x)+x Since g(x)g(x)g(x) satisfies the original equation, for any input, say uuu, we have (g(u))3−3g(u)+u=0(g(u))^3 - 3g(u) + u = 0(g(u))3−3g(u)+u=0. Let's set u=−xu = -xu=−x: (g(−x))3−3g(−x)+(−x)=0  ⟹  [g(−x)]3−3g(−x)=x(g(-x))^3 - 3g(-x) + (-x) = 0 \implies [g(-x)]^3 - 3g(-x) = x(g(−x))3−3g(−x)+(−x)=0⟹[g(−x)]3−3g(−x)=x Now substitute this back into our expression for h(x)h(x)h(x): =−(x)+x=0= -(x) + x = 0=−(x)+x=0 So, h(x)h(x)h(x) also satisfies the implicit equation y3−3y+x=0y^3 - 3y + x = 0y3−3y+x=0.

    • Check the initial condition: h(0)=−g(−0)=−g(0)=−0=0h(0) = -g(-0) = -g(0) = -0 = 0h(0)=−g(−0)=−g(0)=−0=0 So, h(0)=0h(0) = 0h(0)=0.

    Since both g(x)g(x)g(x) and h(x)=−g(−x)h(x) = -g(-x)h(x)=−g(−x) satisfy the same defining equation and the same initial condition, and the problem states that such a function is unique, we must have: g(x)=h(x)  ⟹  g(x)=−g(−x)g(x) = h(x) \implies g(x) = -g(-x)g(x)=h(x)⟹g(x)=−g(−x) This shows that g(x)g(x)g(x) is an odd function.

  5. Use the Odd Function Property to Finalize the Calculation: Since g(x)g(x)g(x) is an odd function, we have the property g(−x)=−g(x)g(-x) = -g(x)g(−x)=−g(x). For x=1x=1x=1, this gives: g(−1)=−g(1)g(-1) = -g(1)g(−1)=−g(1) Now substitute this back into our expression for the integral from Step 2: I=g(1)−g(−1)=g(1)−(−g(1))=g(1)+g(1)=2g(1)I = g(1) - g(-1) = g(1) - (-g(1)) = g(1) + g(1) = 2g(1)I=g(1)−g(−1)=g(1)−(−g(1))=g(1)+g(1)=2g(1)

  6. Conclusion: The value of the integral is 2g(1)2g(1)2g(1). Comparing this with the given options: A: 2g(−1)=2(−g(1))=−2g(1)2g(-1) = 2(-g(1)) = -2g(1)2g(−1)=2(−g(1))=−2g(1) B: 000 C: −2g(1)-2g(1)−2g(1) D: 2g(1)2g(1)2g(1)

    The correct option is D.

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