（y-1）的平方+（2x+y-3）的平方+(2x+y-6)的平方
= (y-1)^2 + (2x+y - 9/2 + 3/2)^2 + (2x+y-9/2 - 3/2)^2
= (y-1)^2 + 2* (2x+y - 9/2)^2 + 2*(3/2)^2
2x+y-9/2 =0
y-1 = 0
x = 7/4
y=1
达到最小值