M = 4x^2 - 12xy + 10y^2 + 4z^2 + 4y + 9
= (2x - 3y)^2 + (y + 2)^2 + 4z^2 + 5
注意式子之中都是平方项,故有最小值.
2x - 3y = 0
y + 2 = 0
y = -2
x = -3