$\int_{0}^{a} \sqrt{\frac{x}{a-x}} d x=$

$\int_{0}^{a} \sqrt{\frac{x}{a-x}} d x=$
  1. $\left(\frac{\pi}{4}\right) a$
  2. $-\pi a$
  3. $2 \pi a$
  4. $\left(\frac{\pi}{2}\right) a$

Solution

We have, $I=\int_0^a \sqrt{\frac{x}{a-x}} d x$ Put, $x=a \sin ^2 \theta \Rightarrow d x=2 a \sin \theta \cos \theta d \theta$ When, $x=Q, \theta=0$ and $x=a, \theta=\pi / 2$ $\begin{aligned} & \therefore \quad I=\int_0^{\pi / 2} \sqrt{\frac{a \sin ^2 \theta}{a\left(1-\sin ^2 \theta\right)}} 2 a \sin \theta \cos \theta d \theta \\ & I=2 a \int_0^{\pi / 2} \frac{\sin \theta}{\cos \theta}(\sin \theta \cos \theta) d \theta \\ & I=a \int_0^{\pi / 2} 2 \sin ^2 \theta d \theta \\ & I=a \int_0^{\pi / 2}(1-\cos 2 \theta) d \theta \\ & I=a\left[\theta-\frac{\sin 2 \theta}{2}\right]_0^{\pi / 2} \\ & I=a\left[\frac{\pi}{2}-0\right]=\left(\frac{\pi}{2}\right) a \end{aligned}$

Asked in: MHT CET 2020 (16 Oct Shift 1)

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