x+y=1，xy=-1/2(x+y)^2(x-y)+2y^2(x+y)=(x+y)[(x+y)(x-y)+2y^2]=(x+y)[x^2-y^2+2y^2]=(x+y)[x^2+y^2]=(x+y)[(x+y)^2-2xy]=1*[1^2-2*(-1/2)]=1+1=2
