若A，B是双曲线8x2-y2=8的两焦点，点C在该双曲线上，且△ABC是等腰三角形，则△ABC的周长为______．
8x2-y2=8化为标准方程：x2-
=1，则c2=1+8=9，即c=3，y2 8
所以焦点A（-3，0），B（3，0）；
△ABC为等腰三角形有三种情况：AC=BC，AB=AC，AB=BC；
（1）AC=BC，这在双曲线中是不可能的，因为双曲线满足|AC-BC|=2a，
显然AC不可能等于BC；
（2）AB=AC，因为AB=6，所以AC=6，由第一定义：|AC-BC|=2a=2，得BC=8或4
所以周长为16或20；
（3）AB=BC，根据对称性，结果同（2）；
所以，△ABC的周长为16或20
故答案为：16或20．
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★Harrods is one of the busiest shops in London. Nearly 35,000 people come to the shop every day and more than 4,000 people work here. Harrods is a big shop. It can be difficult to find what you are looking for. Yesterday an Italian girl, Alice, went to Harrods and bought a watch and some chocolate. She found the things with the help of the store guide.
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| 1. What do people think of Harrods? It's very _______ and _______. 2.  is one of the signs Alice saw in Harrods, it means people mustn't _______ _______. 3. Alice went to the _______ floor to buy the watch and the chocolate. 4. The receipt shows how much Alice paid for the things she bought. The total price was ￡_______. Which note did she pay with? _______ (A or B) A.  B.  5. Suppose Alice was on LG in Harrods, write down two things she might buy. _______ _______ |
