题目

等轴双曲线x2－y2＝a2截直线4x＋5y＝0所得弦长为，则双曲线的实轴长是________． 答案：3 解析 注意到直线4x＋5y＝0过原点，可设弦的一端为(x1，y1)，则有＝. 可得x＝，取x1＝，y1＝－2. ∴a2＝－4＝，|a|＝， ∴2|a|＝3. 书面表达。     假定你是我校高三学生李华，在学校看到如下通知，请根据通知的内容和写作要点向学校写一封申请信：                                HOST FAMILIES WANTED      Teachers and students from Central High School Phoenix will visit our school two weeks later. Host families are wanted to accommodate our friends.      REQUIREMENTS: ◇ Can communicate in English.      ◇ A SEPARATE room is a MUST.      ◇ A private car is preferred.      If interested, please send your APPLICATION IN ENGLISH to the Office of the Principal before THURSDAY. Please consult us for any further information.     写作要点： 1． 你对参与该活动的认识                         2． 你的优势                         3． 作为接待家庭的打算     注意：1. 可根据内容要点适当增加细节，以使行文连贯。                2. 词数100字左右 Dear Sir or Madam,      I am Li Hua from Class One, Senior Three.  _______________________________________________________________________________________________________________________                                                                                                      Yours Sincerely,                                                                                                             Li Hua