题目

That cold January night, I was growing sick of my life in the city I lived. There I was walking home at one in the morning after a tiring practice at the theater. I was having trouble dealing with my part－time job at the bank and my acting at night at the same time. As I walked, I thought seriously about giving up both acting and the life in this awful place.As I walked down the empty street under tall buildings, I felt very small and cold. “Run！” I told myself and began to do so, both to keep warm and keep away from any possible danger.About a block from my flat, I heard a noise behin如图，在?ABCD中，E、F分别是AB、CD上的点且BE=DF，要证明四边形AECF是平行四边形，只需证明________，此时用的判定定理是________．