(-x)(x的2次方-2xy-y的平方）-y(xy+2x的平方-y的平方）
=（-X）[X(X-Y)-Y(X+Y)]-Y[X(X+Y)+Y(X-Y)]
=-X*X(X-Y)+X*Y(X+Y)-Y*X(X+Y)-Y*Y(X-Y)
=-(X-Y)(X*X-Y*Y)
=0