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Please solve RD Sharma Class 12 Chapter 19 Definite Integrals Exercise 19.4 (b) Question 49 maths textbook solution.

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Answer:- Proved

Hints:-  You must know the rules of integration for trignometric functions.

Given:- Prove  \int_{0}^{\pi} x f(\sin ) d x=\frac{\pi}{2} \int_{0}^{\pi} f(\sin x) \cdot d x

Solution : Let I=\int_{0}^{\pi} x f(\sin ) d x                                 ....(1)

                 \begin{aligned} I &=\int_{0}^{\pi}(\pi-x) f(\sin (\pi-x)) d x \\ &=\int_{0}^{\pi}(\pi-x) f(\sin x) d x \end{aligned}        ......(2)

Adding (1) and (2)

               2 I=\int_{0}^{\pi}(x+\pi-x) f(\sin x) d x

               \begin{aligned} &2 I=\pi \int_{0}^{\pi} f(\sin x) d x \\ &I=\frac{\pi}{2} \int_{0}^{\pi} f(\sin x) d x \\ &\therefore \int_{0}^{\pi} f(\sin x) d x=\frac{\pi}{2} \int_{0}^{\pi} f(\sin x) \cdot d x \end{aligned}

Hence proved.

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