f(x)=x^3-27
f'(x)=3x^2>0
故f(x)在R上单调递增
当x=-4时,f(x)取到最小值f(-4)=-91
当x= 4时,f(x)取到最大值f( 4)=37
f(x)=-x^3+3x
f'(x)=-3x^2+3
令f'(x)=0 -> x=1或-1
当x>1时,f'(x)