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