AB=AC+CD
证明：在AB上取点F,使AF=AC,则△ACD≌△AFD,得CD=FD,∠AFD=∠ACD.又 ∠C=2∠B,所以∠AFD=2∠B,因为∠AFD=∠B+∠FDB,所以∠B=∠FDB,则FB=FD,则FB=CD所以AF+FB=AC+CD,即AB=AC+CD.