Q240 / 270 · NDA 2017 (Phase 1) · Mathematics

If $f(\mathrm{x})$ and $\mathrm{g}(\mathrm{x})$ are continuous functions satisfying $f(\mathrm{x})=f(\mathrm{a}-\mathrm{x})$ and $\mathrm{g}(\mathrm{x})+\mathrm{g}(\mathrm{a}-\mathrm{x})=2$, then what is $\int_0^{\mathrm{a}} f(\mathrm{x}) \mathrm{g}(\mathrm{x}) \mathrm{dx}$ equal to?

  • A
    $\quad \int_0^a g(x) d x$
  • Correct answer
    B
    $\int_0^{\mathrm{a}} f(\mathrm{x}) \mathrm{dx}$
  • C
    $\quad 2 \int_0^{\mathrm{a}} f(\mathrm{x}) \mathrm{dx}$
  • D
    0

Solution

$\begin{aligned} & I=\int_0^a f(x) \cdot g(x) d x \\ & I=\int_0^a f(a-x) \cdot g(a-x) \cdot d x \\ & I=\int_0^a f(x) \cdot[2-g(x)] \cdot d x \\ & \{\because f(a-x)=f(x) \text { and } g(a-x)=2 g(x)\} \\ & I=\int_0^a 2 \cdot f(x) \cdot d x-\int_0^a f(x) \cdot g(x) \cdot d x \\ & I=2 \cdot \int_0^a f(x) \cdot-I \\ & \Rightarrow 2 I=2 \int_0^a f(x) d x 2 \\ & \therefore I=\int_0^a f(x) d x \\ & \frac{a}{a} f(x)=\int_0^a f(a-x) \cdot d x \\ & (\because \text { Given } f(x)=f(x))\end{aligned}$

Hence, the correct answer is option: 'B'.

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