（2a-b）*(2a-b）-(a-b)
=4a*a-2ab-2ab+b*b-(a-b)
=4a*a-4ab+b*b-(a-b)
=4a(a-b)+b*b-(a-b)
=(a-b)*(4a-1)-b*b
将a=2分之1,b=—2,代入上式：
=[1/2-(-2)]*(4*1/2-1)-(-2)*(-2)
=2.5*1+4
=2.5+4
=6.5