x(x-1)-(x的平方-y)+2=0
x^2-x-x^2+y+2=0
x-y=2
代数式2分之x的平方+y的平方在-xy
=(x^2+y^2)/2-xy
=(x^2-2xy+y^2)/2
=(x-y)^2/2
=2^2/2
=2