a^2+b^2-ab-a-b+1
=(a-b)^2+(a-1)(b-1)
=[(a-1)-(b-1)]^2+(a-1)(b-1)
令a-1=x,b-1=y,x,y属于R
上式变为：
（x-y)^2+xy
=x^2-xy+y^2
=(x-y/2)^2+3y^2/4>=0