You need to use the following substitution `cos theta = u` , such that:
`cos theta = u=> -sin theta d theta = du =>sin theta d theta = -du`
`int cos^4 theta*sin theta d theta = - int u^4 du`
`- int u^4 du = -(u^5)/5 + c`
Replacing back `cos theta` for` u` yields:
`int cos^4 theta*sin theta d theta = -((cos theta)^5)/5 + c`
Hence, evaluating the indefinite integral, yields `int cos^4 theta*sin theta d theta = -((cos theta)^5)/5 + c`
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