證明：∵?$AD// BC，$?∴?$∠DAE=∠BCF$?
∵?$BF// DE，$?∴?$∠BF A=∠DEC$?
∴?$∠AED=∠CFB$?
在?$△ADE$?和?$△CBF {中}$?
?$\begin {cases}{∠DAE=∠BCF}\\{AE=CF}\\{∠AED=∠CFB}\end {cases}$?
∴?$△ADE≌△CBF(\mathrm {ASA})$?
∴?$AD=CB$?
?$ $?在?$\triangle ABC$?和?$\triangle CDA$?中
?$\begin {cases}{BC=DA}\\{∠ACB=∠CAD}\\{AC=CA}\end {cases}$?
∴?$\triangle ABC≌\triangle CDA(\mathrm {SAS})$?
∴?$AB = CD$?
