由上图滑哗可知EF/CD=BE/BC=BF/BD,另AB/CD=BE/(BC-BE)所以CD/AB=BC/BE-1 另BC/BE=CD/EF 可得CD/AB=(CD-EF)/差答EF ,即AB/CD=EF/(CD-EF)相乘可得CD*EF=AB*CD-AB*EF合并同类得EF(AB+CD)=AB*CD两边同取倒 即1/EF(AB+CD)=1/虚让慧AB*CD同等于1/EF=(AB+CD)/AB*CD 取得证1/EF=1/AB+1/CD,谢谢!