x（x+y)(x-y)-x(x+y)^2的公因式为x(x+y),提公因式得：
原式=x(x+y)[(x-y)-(x+y)]=x(x+y)(x-y-x-y)=-2xy(x+y),
代入已知条件x+y=1,xy=-1/2得
原式=（-2）*（-1/2）=1