If $\int_{o}^{a} \frac{d x}{1+4 x^{2}}=\frac{\pi}{8}$, then $a=$

If $\int_{o}^{a} \frac{d x}{1+4 x^{2}}=\frac{\pi}{8}$, then $a=$
  1. $\frac{1}{2}$
  2. 2
  3. $\frac{1}{4}$
  4. 1

Solution

We have $\int_{0}^{a} \frac{d x}{1+4 x^{2}}=\frac{\pi}{8}$ $\therefore \frac{\pi}{8}=\frac{1}{4} \int_{0}^{a} \frac{d x^{2}+\left(\frac{1}{2}\right)^{2}}{2}=\frac{1}{4} \frac{1}{\left(\frac{1}{2}\right)}\left[\tan ^{-1} \frac{x}{\left(\frac{1}{2}\right)}\right]_{0}^{a}$ $\quad=\frac{1}{2}\left[\tan ^{-1} 2 a-\tan ^{-1} 0\right]$ $\therefore \frac{\pi}{8}=\frac{1}{2} \tan ^{-1} 2 a \Rightarrow \frac{\pi}{4}=\tan ^{-1} 2 a \Rightarrow 2 a=\tan \frac{\pi}{4}=1 \Rightarrow a=\frac{1}{2}$

Asked in: MHT CET 2020 (20 Oct Shift 2)

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