①变形:T1=L/(V1-V2)②变形:T2=L/(V1+V2)代入③得:V2[L/(V1-V2)+L/(V1+V2)]=SV2[1/(V1-V2)+1/(V1+V2)]=S / LV1V2/[(V1-V2)(V1+V2)]=S /(2L)V1V2/(V1^2-V2^2)=S /(2L)分子分母同时除以V2的平方得:(V1/V2)/[(V1/V2)^2-1] = S /(2L)把V1/V2当作一个整体,把S和L得值代入,就可以求出来了