m^2+n^2=4m
则mn分之m^2-n^2
=(m^2-n^2)/mn
=(m+n)(m-n)/mn
=√[(m+n)²(m-n)²]/mn
=√[(m²+2mn+n²)(m²-2mn+n²)]/mn
无法计算下去
题目应该有问题!