a(a-1)-(a²-b)=2
a²-a-a²+b=2
-a+b=2
b-a=2
应该是“ab-(a²+b²)/2”吧
要不然算出来不能化简到常数
ab-(a²+b²)/2
=-[(a²+b²)/2-ab]
=-(a²-2ab+b²)/2
=-(a-b)²/2
=-(-2)²/2
=-2