x^3+x^2y-xy^2-y^3
=(x^3-y^3)+(x^2y-xy^2)
=(x-y)(x^2+xy+y^2)+xy(x-y)
=(x-y)(x^2+2xy+y^2)
=(x-y)(x+y)^2
=0
x+y=28/2=14
x-y=0
x=y=7
矩形面积xy=49