1/EF＝1/AC＋1/BD证明：∵AC∥EF∥BD∴EF/AC＝FB/AB，EF/BD＝AF/AB两式相加可得：EF/AC＋EF/BD＝(AF＋BF)/AB＝1∴1/AC＋1/BD＝1/EF1/EF=1/20+1/10EF=20/3
