Question
If $$\int {{{\sin }^3}x\,{{\cos }^5}x} \,dx = A\,{\sin ^4}x + B\,{\sin ^6}x + C\,{\sin ^8}x + D.$$
Then :
A.
$$A = \frac{1}{4},\,B = - \frac{1}{3},\,C = \frac{1}{8},\,D\, \in \,R$$
B.
$$A = \frac{1}{8},\,B = \frac{1}{4},\,C = \frac{1}{3},\,D\, \in \,R$$
C.
$$A = 0,\,B = - \frac{1}{6},\,C = \frac{1}{8},\,D\, \in \,R$$
D.
None of these
Answer :
$$A = \frac{1}{4},\,B = - \frac{1}{3},\,C = \frac{1}{8},\,D\, \in \,R$$
Solution :
$$\eqalign{
& I = \int {{{\sin }^3}x.{{\cos }^5}x} \,dx \cr
& {\text{Put }}\sin \,x = t \Rightarrow \cos \,x\,dx = dt \cr
& I = \int {{{\sin }^3}x.{{\cos }^4}x} .\cos \,x\,dx \cr
& \,\,\,\,\, = \int {{t^3}{{\left( {1 - {t^2}} \right)}^2}dt} \cr
& \,\,\,\,\, = \int {\left( {{t^3} - 2{t^5} + {t^7}} \right)} dt \cr
& \,\,\,\,\, = \frac{1}{4}{t^4} - \frac{2}{6}{t^6} + \frac{1}{8}{t^8} + D \cr
& \,\,\,\,\, = \frac{1}{4}{\sin ^4}x - \frac{1}{3}{\sin ^6}x + \frac{1}{8}{\sin ^8}x + D \cr} $$