已知：m+1/m=3 两边平方m^2+2+1/m^2=9,m^2+1/m^2=7 (m-1/m)^2=m^2-2+1/m^2=7-2=5 m-1/m=±√5 m^2-1/m^2=(m+1/m)(m-1/m)=3(m-1/m)＝±3√5 楼主的答案是正解.
