2(a²+b²)-2(ab+a+b-1)=2a²+2b²-2ab-2a-2b+2=(a²-2ab+b²)+(a²-2a+1)+(b²-2b+1)=(a-b)²+(a-1)²+(b-1)²平方大于等于0所以(a-b)²+(a-1)²+(b-1)²...