设 p=sin(nx),q=(cosx)^n 则p'=ncos(nx),q'=cos(x+nπ/2)∴y'=p'q+pq' =ncos(nx)·(cosx)^n+sin(nx)·cos(x+nπ/2)
n*cosnx*(cosx)^n+sinnx*(-sinx)n*(cosx)^(n-1)
