2(x²-xy+y²)-2(x+y-1)=(x²-2x+1)+(y²-2y+1)+(x²-2xy+y²)=(x-1)²+(y-1)²+(x-y)²平方大于等于0所以(x-1)²+(y-1)²+(x-y)²>=0所以2(x²-xy+y²)-2...