Question
$$\int {\frac{{dx}}{{\sin \,x\left( {3 + {{\cos }^2}x} \right)}}} $$ is equal to :
A.
$$\log \left| {{y^2} - 1} \right| - {\tan ^{ - 1}}y + C$$
B.
$${\tan ^{ - 1}}\frac{y}{{\sqrt 3 }} + C$$
C.
$$\log \left| {\frac{{y - 1}}{{y + 1}}} \right| + C$$
D.
$$\frac{1}{4}\log \left| {\frac{{y - 1}}{{y + 1}}} \right| - \frac{1}{{4\sqrt 3 }}{\tan ^{ - 1}}\frac{y}{{\sqrt 3 }} + C$$
Answer :
$$\frac{1}{4}\log \left| {\frac{{y - 1}}{{y + 1}}} \right| - \frac{1}{{4\sqrt 3 }}{\tan ^{ - 1}}\frac{y}{{\sqrt 3 }} + C$$
Solution :
$$\eqalign{
& \int {\frac{{dx}}{{\sin \,x\left( {3 + {{\cos }^2}x} \right)}}} \cr
& = \int {\frac{{\sin \,x\,dx}}{{{{\sin }^2}\,x\left( {3 + {{\cos }^2}x} \right)}}} \cr
& = \int {\frac{{\sin \,x\,dx}}{{\left( {1 - {{\cos }^2}x} \right)\left( {3 + {{\cos }^2}x} \right)}}} \cr
& = \int {\frac{{dy}}{{\left( {{y^2} - 1} \right)\left( {{y^2} + 3} \right)}}\,\,\,\,\,\left( {{\text{Putting }}\cos \,x = y} \right)} \cr
& = \frac{1}{4}\int {\left[ {\frac{1}{{{y^2} - 1}} - \frac{1}{{{y^2} + 3}}} \right]dy} \cr
& = \frac{1}{4}\log \left| {\frac{{y - 1}}{{y + 1}}} \right| - \frac{1}{{4\sqrt 3 }}{\tan ^{ - 1}}\frac{y}{{\sqrt 3 }} + C \cr} $$