当x=0时,f(x)的导数为0
当x0时,f(x)的导数为g’(x)*cos(1/x)+g(x)*[-sin(1/x)]*(-1/x^2)=g’(x)*cos(1/x)+g(x)*[sin(1/x)]*(1/x^2)
所以
当x=0时f'(0)=0
当x0时,f'(0)=0
故f'(0)=0g(x)*[sin(1/x)]*(1/x^2)=g(x)/x*[sin(1/x)/x当x趋于0时，g(x)/x等价于g'(0)，sin(1/x)/x再求导，