此题应为：已知x（x-1)-(x^2-y)= -2 求（x^2+y^2)/2-xy的值.
由 x(x-1)-(x^2-y)= -2
展开 得 x^2-x-x^2+y= -2
x-y=2
所以 (x^2+y^2)/2-xy
=x^2/2+y^2/2-2xy/2
=(x-y)^2/2
将x-y=2整体代入
得 =(x-y)^2/2
=4/2
=2