a2-2a+b2+2b+2
=a2-2a+1+（b2+2b+1）
=（a-1）^2+(b+1)^2
a=1,b=-1
a-b=2试说明：不管a、b取何值2a2+5b2-2ab+2a-4b+5的值始终为正数2a2+5b2-2ab+2a-4b+5=（a2+2a+1）+(a2-2ab+b2)+4(b2-b+1/4)+3=(a+1)^2+(a-b)^2+4(b-1/2)^2+3>=3