展开(mx+y)(x-y)=mx^2+(m-1)x*y-y^2=2x^2+nxy-y^2所以 m=2 m-1=nn=1
(mx+y)(x-y)=2x^2+nxy-y^2mx^2+(1-m)xy-y^2=2x^2+nxy-y^2m=2(1-m)=nn=-1