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  • Need solution for RD Sharma maths Class 12 Chapter 19 Definite Integrals Exercise 19.4 (b) Question 16 textbook solution.

Need solution for RD Sharma maths Class 12 Chapter 19 Definite Integrals Exercise 19.4 (b) Question 16 textbook solution.

Answers (1)

Answer:- \frac{\alpha \pi}{\sin \alpha}

Hints:-  You must know the integration rules of trigonometric functions and its limits.

Given:-  \int_{0}^{\pi} \frac{x}{1+\cos \alpha \sin x} d x, \quad 0<\alpha<\pi

Solution : \int_{0}^{\pi} \frac{x}{1+\cos \alpha \sin x} d x                                                 ....(1)

               \begin{aligned} &I=\int_{0}^{\pi} \frac{(\pi-x)}{1+\cos \alpha \sin (\pi-x)} d x \\ &I=\int_{0}^{\pi} \frac{(\pi-x)}{1+\cos \alpha \sin x} d x \end{aligned}                           .....(2)

Adding (1) and (2)

              \begin{aligned} &2 I=\pi \int_{0}^{\pi} \frac{d x}{1+\cos \alpha \sin x} \\ &2 I=\pi \int_{0}^{\pi} \frac{\sec ^{2} \frac{\pi}{2} d x}{\left(1+\tan ^{2} \frac{x}{2}\right)+2 \cos \alpha \tan \frac{x}{2}} \end{aligned}

Put \tan\frac{x}{2}=t

             \begin{aligned} &\sec ^{2} \frac{\pi}{2} d x=2 d t \\ &2 I=\pi \int_{0}^{\pi} \frac{2 d t}{1+t^{2}+2 t \cos \alpha} \\ &2 I=2 \pi \int_{0}^{\pi} \frac{d t}{(t+\cos \alpha)^{2}+\sin ^{2} \alpha} \end{aligned}

            \begin{aligned} &I=\frac{\pi}{\sin \alpha}\left[\tan ^{-1}\left(\frac{t+\cos \alpha}{\sin \alpha}\right)\right]_{0}^{\infty} \\ &=\frac{\pi}{\sin \alpha}\left[\tan ^{-1}(\infty)-\tan ^{-1}(\cot \alpha)\right] \\ &=\frac{\pi}{\sin \alpha}\left[\frac{\pi}{2}-\left(\frac{\pi}{2}-\alpha\right)\right] \\ &=\frac{\alpha \pi}{\sin \alpha} \end{aligned}

 

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