m二次方- 5m-1=0,
m-5-1/m=0
m-1/m=5
m^2+1/m^2-2=25
m^2+1/m^2=27
2（m）二次方-5m+1/m二次方
=m^2-5m+(m^2+1/m^2)
=1+27
=28