[tex]\it [AB]\ \equiv\ [DE]\ \ \ \ \ (1)\\ \\ \[[BF]\equiv[CD]\Rightarrowbf=CD\bigg|_{+FC} \Rightarrow BC=DF \Rightarrow \[[BC]\ \equiv\ [DF]\ \ \ \ \ \ (2)\\ \\ \\ (1).\ (2)\ \stackrel{C.I.}{\Longrightarrow}\ \Delta ABC\ \equiv\ \Delta E DF \Rightarrow \[[AC]\ \equiv\ [EF][/tex]