FEC∽AFG
EF=6 EC=3
FC=3根号5
GF=6
GF:FC=AF:EC(3)=6:3根号5
AF=6/5根号5
AC=21/5根号5
AGF∽ABC
AC:AF=BC:GF=21/5:6/5
BC=21
方法二
EFC∽AGF∽GDB
EC:DG=EF:BD
BD=12
BC=12+6+3=21
FEC∽AFG
EF=6 EC=3
FC=3根号5
GF=6
GF:FC=AF:EC(3)=6:3根号5
AF=6/5根号5
AC=21/5根号5
AGF∽ABC
AC:AF=BC:GF=21/5:6/5
BC=21
方法二
EFC∽AGF∽GDB
EC:DG=EF:BD
BD=12
BC=12+6+3=21