f'(x)=-x^2+4ax-3a^2=-(x-3a)(x-a) 负无穷到a单调减,a到3a单调增,3a到正无穷单调减f'(x)对称轴为2a当a<1/2时,3a当a>1/2时,a+1<3a恒有|f'(x)| <4a-4