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Definite Integration question

2021 · 24 Feb · Shift 2 · Q27
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  5. /2021 · 24 Feb · Shift 2 · Q27

Definite Integration question

2021 · 24 Feb · Shift 2 · Q27

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
Let f(x) be a differentiable function defined on [0, 2] such that f'(x) = f'(2 −-− x) for all x ∈\in∈ (0, 2), f(0) = 1 and f(2) = e2. Then the value of ∫02f(x)dx\int\limits_0^2 {f(x)} dx0∫2​f(x)dx is :
  1. A
    1 + e2
  2. B
    2(1 + e2)
  3. C
    1 −-− e2
  4. D
    2(1 −-− e2)
View written solutionFree

Correct answer: A

  1. We are given

    \quad \text{for all } x\in(0,2),$$ with $$f(0)=1,\qquad f(2)=e^2.$$
  2. Define a new function g(x)=f(x)+f(2−x).g(x)=f(x)+f(2-x).g(x)=f(x)+f(2−x). Differentiate it: g′(x)=f′(x)−f′(2−x).g'(x)=f'(x)-f'(2-x).g′(x)=f′(x)−f′(2−x). Here, derivative of f(2−x)f(2-x)f(2−x) is −f′(2−x)-f'(2-x)−f′(2−x) by chain rule.

  3. Using the given condition f′(x)=f′(2−x)f'(x)=f'(2-x)f′(x)=f′(2−x), we get g′(x)=f′(x)−f′(2−x)=0.g'(x)=f'(x)-f'(2-x)=0.g′(x)=f′(x)−f′(2−x)=0. Hence, g(x)g(x)g(x) is constant on [0,2][0,2][0,2].

  4. Find this constant using x=0x=0x=0: g(0)=f(0)+f(2)=1+e2.g(0)=f(0)+f(2)=1+e^2.g(0)=f(0)+f(2)=1+e2. Therefore, f(x)+f(2−x)=1+e2for all x∈[0,2].f(x)+f(2-x)=1+e^2 \quad \text{for all } x\in[0,2].f(x)+f(2−x)=1+e2for all x∈[0,2].

  5. Integrate both sides from 000 to 222: ∫02(f(x)+f(2−x)) dx=∫02(1+e2) dx.\int_0^2 \big(f(x)+f(2-x)\big)\,dx=\int_0^2 (1+e^2)\,dx.∫02​(f(x)+f(2−x))dx=∫02​(1+e2)dx.

  6. Now, ∫02f(2−x) dx=∫02f(x) dx\int_0^2 f(2-x)\,dx=\int_0^2 f(x)\,dx∫02​f(2−x)dx=∫02​f(x)dx by substitution u=2−xu=2-xu=2−x. So the left side becomes 2∫02f(x) dx.2\int_0^2 f(x)\,dx.2∫02​f(x)dx.

  7. Hence, 2∫02f(x) dx=2(1+e2).2\int_0^2 f(x)\,dx = 2(1+e^2).2∫02​f(x)dx=2(1+e2). Therefore, ∫02f(x) dx=1+e2.\int_0^2 f(x)\,dx = 1+e^2.∫02​f(x)dx=1+e2.

  8. Checking options:

    • A: 1+e21+e^21+e2 ✓
    • B: 2(1+e2)2(1+e^2)2(1+e2) ✗
    • C: 1−e21-e^21−e2 ✗
    • D: 2(1−e2)2(1-e^2)2(1−e2) ✗

So the correct option is A.

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