因m+m分之1=根号5 m>0 (m+1/m)=5 (m+1/m)^2=25 m^2 +2 +(1/m)^2=25 m^2 -2 +(1/m)^2=21 (m-1/m)^2=21 m>（1/m） (m-1/m)=根号下21 m