Explanation
$$\displaystyle \int_{0}^{\pi/ 4} \dfrac {\sin \theta + \cos \theta}{9 + 16\sin 2\theta} d\theta$$
Let $$\sin \theta - \cos \theta = t$$
$$\therefore (\cos \theta + \sin \theta) d\theta = dt$$
$$= \displaystyle \int_{-1}^{0} \dfrac {dt}{9 + 16(1 – t^{2})} (\because t^{2} = 1 – 2\cdot \sin \theta \cdot \cos \theta)$$
$$= \displaystyle \int_{-1}^{0} \dfrac {dt}{25 – 16t^{2}}$$
$$= \dfrac {1}{16} \displaystyle \int_{-1}^{0} \dfrac {dt}{\left (\dfrac {5}{4}\right )^{2} – t^{2}}$$
$$= \dfrac {1}{16}\times \dfrac {1}{2\times \dfrac {5}{4}}\left [ln \left |\dfrac {\dfrac {5}{4} +t}{\dfrac {5}{4} – t}\right |\right ]_{-1}^{0}$$
$$\left (\because \displaystyle \int \dfrac {1}{a^{2} – x^{2}} dx = \dfrac {1}{2a} ln \left |\dfrac {a + x}{a – x}\right | + c\right )$$
$$= \dfrac {1}{40} \left [ln |1| - ln \left |\dfrac {\dfrac {1}{4}}{\dfrac {9}{4}}\right |\right ]$$
$$= \dfrac {1}{40} ln 9 = \dfrac {ln 3}{20}$$
$$\therefore \displaystyle \int_{0}^{\pi/ 4} \dfrac {\sin \theta + \cos \theta}{9 + 16 \sin 2\theta} d\theta = \dfrac {ln 3}{20}$$.
$$\int_{0}^{\dfrac{\pi}{2}} \sqrt{cos x}sin^5 x dx$$ $$\int_{0}^{\dfrac{\pi}{2}}\sqrt{cos x}(sin^4x)sin x dx$$ $$-\int_{0}^{\dfrac{\pi}{2}}\sqrt{cos x}(1-cos^2 x)^2d cos x$$ $$-\left [ \int_{0}^{\dfrac{\pi}{2}}\sqrt{cos x} dcos x+\int_{0}^{\dfrac{\pi}{2}}\sqrt{cos x} cos^4 x dx - 2\int_{0}^{\dfrac{\pi}{2}}\sqrt{cos x} cos^2 x dx \right ]$$ $$-\left [ \dfrac{2}{3} cos^{\dfrac{3}{2}} x \int_{0}^{\dfrac{\pi}{2}}+\dfrac{2}{11} cos^{\dfrac{11}{2}}x\int_{0}^{\dfrac{\pi}{2}}-2(\dfrac{2}{7}) cos^{\dfrac{7}{2}}x \int_{0}^{\dfrac{\pi}{2}} \right ]$$ $$=-\left [\dfrac{2}{3} (-1)+\dfrac{2}{11}(-1)-2(\dfrac{2}{7})(-1)\right ]$$ $$=\dfrac{64}{231}$$
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