你的-2/1是不是 负2分之1 啊？-1/2x^3y-1/2xy^3+x^2y^2-1/2xy(x^2-2xy+y^2）-1/2xy（x-y)^2-1/2×8×（1/2）^2最后答案=-1
原式=-1/2xy（x^2+y^2-2xy) =-1/2xy(x-y)^2 =-1/2*8*(1/2)^2 =-1
