5x^2 - 2xy + y^2 - 4x + 1= (x^2 - 2xy + y^2) + (4x^2 - 4x + 1)= (x - y)^2 + (2x - 1)^2= 0平方和等于0 ,则每个加式等于 0x - y = 02x - 1 = 0x = y = 1/2x^2*y = 1/8