题目

It was an extremely cold December night, and I had just settled into the living room sofa with a book when I heard a sharp knocking on the front door. The sound frightened me, mainly because it was 8:45 PM, quite dark outside, and I was alone. My husband had left two days before for a meeting.I sat thinking for a moment about who could be at my door at that hour. I live on a really quiet street on the North Shore about a block off the main road. Hours can go by without a car or pedestrian(行人)passing in front of my home, a small house that's more than 100 years old.Another moment passed. Mo（2005•连云港）如图，在△ABC中，∠ACB=90°，DE是△ABC的中位线，点F在AC延长上，且CF=AC．求证：四边形ADEF是等腰梯形．