Question
If $$\int {\frac{{\sin \,x}}{{\sin \left( {x - \alpha } \right)}}dx = Ax + B\log \,\sin \left( {x - \alpha } \right), + C,} $$ then value of $$\left( {A,\,B} \right)$$ is-
A.
$$\left( { - \cos \,\alpha ,\,\sin \,\alpha } \right)$$
B.
$$\left( {\cos \,\alpha ,\,\sin \,\alpha } \right)$$
C.
$$\left( { - \sin \,\alpha ,\,\cos \,\alpha } \right)$$
D.
$$\left( {\sin \,\alpha ,\,\cos \,\alpha } \right)$$
Answer :
$$\left( {\cos \,\alpha ,\,\sin \,\alpha } \right)$$
Solution :
$$\eqalign{
& \int {\frac{{\sin \,x}}{{\sin \left( {x - \alpha } \right)}}dx} = \int {\frac{{\sin \,\left( {x - \alpha + \alpha } \right)}}{{\sin \left( {x - \alpha } \right)}}dx} \cr
& = \int {\frac{{\sin \left( {x - \alpha } \right)\cos \,\alpha + \cos \left( {x - \alpha } \right)\sin \,\alpha }}{{\sin \left( {x - \alpha } \right)}}dx} \cr
& = \int {\left\{ {\cos \,\alpha + \sin \,\alpha \,\cot \left( {x - \alpha } \right)} \right\}dx} \cr
& = \left( {\cos \,\alpha } \right)x + \left( {\sin \,\alpha } \right)\log \,\sin \,\left( {x - \alpha } \right) + C \cr
& \therefore A = \cos \,\alpha ,\,\,\,B = \sin \,\alpha \cr} $$