a^2+b^2-(ab+a+b-1) =a^2/2-a+1/2+b^2/2-b+1/2+a^2/2-ab+b^2/2 =(a-1)^2/2+(b-1)^2/2+(a-b)^2/2>=0 所以a^2+b^2>=ab+a+b-1 (当a=b=1时取等号)