f(x)以T为周期,则f(x)·f'(x)是以T为周期的函数吗?证明:f(x)以T为周期故f'(x)是以T为周期,故f(x+T)·f'(x+T)=f(x)·f'(x)但是举例:f(x)=sinx,f'(x)=cosx,两者周期为2π,但f(x)·f'(x)=sinx·cosx=1/2sin2x,周期为π哪个对?