求函数y=-1/4(x四次方-4x立方+3)的单调区间和极值

y=-1/4(x^4-4x^3+3)
求导
y'
=-1/4(4x^3-12x^2)
=-(x^3-3x^2)
=-x^2(x-3)
①当x>3时
x-3>0
y'