m^2+n^2-p^2=0
m^2+n^2=p^2
所以,p^2=m^2+n^2>=2mn
p^2/(m+n)^2
=p^2/(m^2+n^2+2mn)
=p^2/(p^2+2mn)
>=p^2/(p^2+p^2)
=1/2
即p/(m+n)>=根号(1/2)=根号2/2
所以,最小值是根号2/2.