Question
The value of $$\int {\frac{{\sin \,x}}{{\sin \,4x}}} dx$$ is :
A.
$$\frac{1}{4}\log \left| {\frac{{\sin \,x - 1}}{{\sin \,x + 1}}} \right| - \frac{1}{{\sqrt 2 }}\log \left| {\frac{{\sqrt 2 \,\sin \,x - 1}}{{\sqrt 2 \,\sin \,x + 1}}} \right| + C$$
B.
$$\frac{1}{8}\log \left| {\frac{{\cos \,x - 1}}{{\cos \,x + 1}}} \right| - \frac{1}{{2\sqrt 2 }}\log \left| {\frac{{\sqrt 2 \,\cos \,x - 1}}{{\sqrt 2 \,\cos \,x + 1}}} \right| + C$$
C.
$$\frac{1}{8}\log \left| {\frac{{\sin \,x - 1}}{{\sin \,x + 1}}} \right| - \frac{1}{{4\sqrt 2 }}\log \left| {\frac{{\sqrt 2 \,\sin \,x - 1}}{{\sqrt 2 \,\sin \,x + 1}}} \right| + C$$
D.
None of these
Answer :
$$\frac{1}{8}\log \left| {\frac{{\sin \,x - 1}}{{\sin \,x + 1}}} \right| - \frac{1}{{4\sqrt 2 }}\log \left| {\frac{{\sqrt 2 \,\sin \,x - 1}}{{\sqrt 2 \,\sin \,x + 1}}} \right| + C$$
Solution :
$$\eqalign{
& I = \int {\frac{{\sin \,x\,dx}}{{4\,\sin \,x\,\cos \,x\,\cos \,2x}}} \cr
& = \frac{1}{4}\int {\frac{{\cos \,x\,dx}}{{\left( {1 - {{\sin }^2}x} \right)\left( {1 - 2\,{{\sin }^2}x} \right)}}} \cr
& = \frac{1}{4}\int {\frac{{dt}}{{\left( {1 - {t^2}} \right)\left( {1 - 2{t^2}} \right)}}\,\,\,\,\,\,\,\,\,\,\left[ {t = \sin \,x} \right]} \cr
& = \frac{1}{4}\int {\left( {\frac{2}{{1 - 2{t^2}}} - \frac{1}{{1 - {t^2}}}} \right)dt} \cr
& = \frac{1}{4}\left\{ {\frac{2}{{2\sqrt 2 }}\log \left| {\frac{{1 + \sqrt 2 t}}{{1 - \sqrt 2 t}}} \right| - \frac{1}{2}\log \left| {\frac{{1 + t}}{{1 - t}}} \right|} \right\} + C \cr
& = \frac{1}{8}\log \left| {\frac{{\sin \,x - 1}}{{\sin \,x + 1}}} \right| - \frac{1}{{4\sqrt 2 }}\log \left| {\frac{{\sqrt 2 \,\sin \,x - 1}}{{\sqrt 2 \,\sin \,x + 1}}} \right| + C \cr} $$