MCQ
If $\int_0^{\frac{\pi}{4}} \frac{\sin ^2 x}{1+\sin x \cos x} d x=\frac{1}{a} \log _e\left(\frac{a}{3}\right)+\frac{\pi}{b \sqrt{3}}$, where a, $\mathrm{b} \in \mathrm{N}$, then $\mathrm{a}+\mathrm{b}$ is equal to ....................
  • A
    $6$
  • $8$
  • C
    $4$
  • D
    $1$

Answer

Correct option: B.
$8$
b
$ \int_0^{\frac{\pi}{2}} \frac{\sin ^2 x}{1+\frac{1}{2} \sin 2 x} d x=\int_0^{\frac{\pi}{4}} \frac{1-\cos 2 x}{2+\sin 2 x} d x $

$ \int \frac{1}{2+\sin 2 x}-\int \frac{\cos 2 x}{2+\sin 2 x}$

$ \left.\left(\mathrm{I}_1\right)-\mathrm{I}_2\right) $

$ \left(\mathrm{I}_1\right)=\int \frac{\mathrm{dx}}{2+\frac{2 \tan \mathrm{x}}{1+\tan ^2 \mathrm{x}}} $

$ \int_0^{\frac{\pi}{4}} \frac{\sec ^2 \mathrm{xdx}}{2 \tan ^2 \mathrm{x}+2 \tan \mathrm{x}+2}$

$ \frac{1}{2} \int_0^1 \frac{d t}{\left(t+\frac{1}{2}\right)^2+\frac{3}{4}}=\frac{\pi}{6 \sqrt{3}} $

$ I_2=\int_0^{\pi / 4} \frac{\cos 2 x}{2+\sin 2 x} d x=\frac{1}{2}\left(\ln \frac{3}{2}\right) $

$ I_1-I_2=\frac{1}{\sqrt{3}} \frac{\pi}{6}+\frac{1}{2} \ln \frac{2}{3} $

$ \Rightarrow a=2, b=6$

Ans. $8$

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