原式=m/(m n) m/(m-n)-n^2/(m^2-n^2) =(m^2-mn m^2 mn)/(m^2-n^2) -n^2/(m^2-n^2) =(2m^2-n^2)/(m^2-n^2) 因为m/n=5/3,所以m=5n/3,所以原式=41/16