题目

There is a long history for Chinese to drink tea. And it had been widely spread in China for more than 1,000 years    46   first appearing in Europe. Tea drinking is popular throughout history. Many people now go to expensively decorated teahouses    47    (talk) business. However, tea drinking is very complex. Many things contribute to an     48   (enjoy) experience. It starts with surroundings. In the past, it took place in a setting    49    “spring water runs on marble”. At present, to create such an atmosphere, teahouses    50    (decorate) with t如图，在Rt△ABC中，∠ACB=90°，以AC为直径作⊙O交AB于点D，E为BC的中点，连接DE并延长交AC的延长线于点F． （1）求证：DE是⊙O的切线；（2）若CF=2，DF=4，求⊙O直径的长．