y = x^3 - 6x^2 + 9x - 4
令 y ' = 3x^2 - 12x + 9 = 0
得 x = 1 , 3
y" = 6x -12,
y"(1) = 6 -12 < 0 ,f(x)取极大值 0
y"(3) = 18 -12 > 0 ,f(x)取极小值 -4