m²-5m+1=0
m²+1=5m,方程式兩邊同時除以m,得到
m+1/m=5
（m+1/m）^3=m^3+1/m^3+3(m+1/m),所以
m^3+1/m^3=(m+1/m)^3-3(m+1/m)=5^3-3×5=110
(m+1/m)^2=m^2+1/m^2+2,所以,m^2+1/m^2=(m+1/m)^2-2=5^2-2=23
(m-1/m)^2=m^2+1/m^2-2=23-2=21,
m-1/m=±√21
多謝!