x(x-1)-(x²-1)=-2
x²-x-x²+1=-2
-x+1=-2
x=2+1
x=3
(x²+y²)/2-xy
=(x²-2xy+y²)/2
=(x-y)²/2
没有y,解不出来!x(x-1)-(x²-y)=-2
对不起，这是原题x(x-1)-(x²-y)=-2

x²-x-x²+y=-2
-x+y=-2
x-y=2

(x²+y²)/2-xy
=(x²-2xy+y²)/2
=(x-y)²/2
=2²/2
=2