如图.四边形ABCD中,AC与BD交于G,作AE⊥BD于E,CF⊥BD于F.S⊿ABD=(1/2)BD*AE; sina=AE/AG,则:AE=AG*sina.∴S⊿ABD=(1/2)BD*AG*sina;同理:S⊿CBD=(1/2)BD*CG*sina.S四边形ABCD=S⊿ABD+S⊿CBD=(1/2)BD*AG*sina+(1/2...