mn=3,m>0
n>0
m-2n=5
m=2n+5
mn=3
n(2n+5)=3
2n^2+5n-3=0
(2n-1)(n+3)=0
n=1/2或n=-3（舍去）
m=2n+5
=2*1/2+5
=5
m^2+4n^2
=5^2+4*(1/2)^2
=25+1
=26
m+2n
=5+2*1/2
=5+1
=6
n^2-m^2
=(1/2)^2-5^2
=1/4-25
=-99/4