Question
What is $$\int {{e^{\ln \,x}}\sin \,x\,dx} $$ equal to ?
Where $$'c’$$ is a constant of integration.
A.
$${e^{\ln \,x}}\left( {\sin \,x - \cos \,x} \right) + c$$
B.
$$\left( {\sin \,x - x\,\cos \,x} \right) + c$$
C.
$$\left( {x\,\sin \,x + \cos \,x} \right) + c$$
D.
$$\left( {\sin \,x + x\,\cos \,x} \right) - c$$
Answer :
$$\left( {\sin \,x - x\,\cos \,x} \right) + c$$
Solution :
$$\eqalign{
& {\text{Let }}I = \int {{e^{\ln \,x}}\sin \,x\,dx} \cr
& = \int {x\,\sin \,x\,dx} \,\,\,\,\,\,\,\,\left( {\because \,{e^{\log \,a}} = a} \right) \cr
& = - x\,\cos \,x + \int {1.\cos \,x\,dx} \cr
& = \left( {\sin \,x - x\,\cos \,x} \right) + c \cr} $$