合并多项式,（m/m+n)+(m/m-n)-(m^2/m^2-n^2)
=2m^2/(m^2-n^2)-(m^2/m^2-n^2)
=m^2/(m^2-n^2)
将 m=5n/3代入得,（5n/3)^2/[(5n/3)^2-n^2]=(25/9)/[(25/9)-1]=25/16