Question
$$\int {\frac{{dx}}{{\cos \,x + \sqrt 3 \sin \,x}}} $$ equal to :
A.
$$\log \,\tan \left( {\frac{x}{2} + \frac{\pi }{{12}}} \right) + C$$
B.
$$\log \,\tan \left( {\frac{x}{2} - \frac{\pi }{{12}}} \right) + C$$
C.
$$\frac{1}{2}\log \,\tan \left( {\frac{x}{2} + \frac{\pi }{{12}}} \right) + C$$
D.
$$\frac{1}{2}\log \,\tan \left( {\frac{x}{2} - \frac{\pi }{{12}}} \right) + C$$
Answer :
$$\frac{1}{2}\log \,\tan \left( {\frac{x}{2} + \frac{\pi }{{12}}} \right) + C$$
Solution :
$$\eqalign{
& I = \int {\frac{{dx}}{{\cos \,x + \sqrt 3 \sin \,x}}} \cr
& \Rightarrow I = \int {\frac{{dx}}{{2\left[ {\frac{1}{2}\cos \,x + \frac{{\sqrt 3 }}{2}\sin \,x} \right]}}} \cr
& \Rightarrow I = \frac{1}{2}.\int {\frac{{dx}}{{\left[ {\sin \frac{\pi }{6}\cos \,x + \cos \frac{\pi }{6}\sin \,x} \right]}}} \cr
& \Rightarrow I = \frac{1}{2}.\int {\frac{{dx}}{{\sin \left( {x + \frac{\pi }{6}} \right)}}} \cr
& \Rightarrow I = \frac{1}{2}.\int {{\text{cosec}}\left( {x + \frac{\pi }{6}} \right)dx} \cr
& \because \,\int {{\text{cosec}}\,x\,dx} = \log \left| {\left( {\tan \frac{x}{2}} \right)} \right| + C \cr
& \therefore \,I = \frac{1}{2}.\log \,\tan \left( {\frac{x}{2} + \frac{\pi }{{12}}} \right) + C \cr} $$