初中数学题已知x(x
已知 x(x-1)-(x^2-y)=-3, 求(x^2+y^2)÷2-xy 的值
x｛x-1｝-｛x＾2-y｝＝-3 即x^2-x-x^2+y=-3 -x+y=-3 x-y=3 而x＾2+y＾2=(x-y)^2+2xy 所以｛x＾2+y＾2｝/2 -xy =[(x-y)^2+2xy]/2-xy =(x-y)^2/2+xy-xy =(x-y)^2/2 =9/2