证明：∵ef在bd上，bf=de ∴b范市掌丰f-ef=de-ef即be=df ∵ab=cd,a首点血过e=cf,be=df ∴△abe全等于△cdf ∴∠360问答abe=∠cdf 设ac与bd相交的点为o ∵ab=cd,∠abe=∠cdf,∠aob=∠cod ∴△abo全等于△cdo ∴bo=do,ao=co 即ac与bd相互平分