(m+n)^2=7
m^2+n^2+2mn=7
（m-n)^2=3
m^2+n^2-2mn=3
4mn=4
mn=1
m^2+n^2=5
(m^2+n^2)^2=m^4+n^4+2m^2n^2=25
m^4+n^4=25-2=23