m^2-3m+1=0,n^2-3n+1=0
m,n是方程x^2-3x+1=0的两根
m+n=3,mn=1
1/m^2+1/n^2=(m^2+n^2)/(mn)^2=[(m+n)^2-2mn]/(mn)^2
=(9-2)/1
=7