m^2+5m+1=0
m不为0,则
m+5+(1/m)=0
m+(1/m)=-5
[m+(1/m)]^2=25
m^2+1/m^2+2*m*(1/m)=25
m^2+1/m^2+2=25
m^2+1/m^2=23
m^2+1/m^2-8=15