当AD=1,BC=7,且BE平分∠ABC时求EF的长
EFC-EGD全等,所有DG=CF,BF=AG= AD+ CF
<EBF = 1/2 <ABF
<EFC = <ABF = <EBF + <FEB
所以<EBF=<BEF,BEF为等腰,BF= EF= AD+CF = AD+BC-BF
BF=1/2 (AD+BC) = 4