∫ sinx⋅sin( cosx ) dx (1)
Consider cosx=t .
Therefore,
cosx=t
Differentiating both sides,
−sinxdx=dt
By substituting sinxdx=dt in equation (1), we get:
∫ sinx⋅sin( cosx ) dx=− ∫ ( sint )dt =−[ −cost ]+C =cos( cosx )+C
Thus, the required integral is I=cos( cosx )+C .