问题
选择题
设F1，F2是双曲线
|
答案
由题意得，△PF1F2是直角三角形，
由勾股定理得 （2c）2=|PF1|2+|PF2|2=|PF1-PF2|2-2|
||PF1
|=4a2-4ac，∴c2-ac-a2=0，e2-e-1=0 且e＞1，PF2
解方程得e=
，1+ 5 2
故选 A．
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设F1，F2是双曲线
|
由题意得，△PF1F2是直角三角形，
由勾股定理得 （2c）2=|PF1|2+|PF2|2=|PF1-PF2|2-2|
||PF1
|=4a2-4ac，∴c2-ac-a2=0，e2-e-1=0 且e＞1，PF2
解方程得e=
，1+ 5 2
故选 A．
| 选择方框中所给词组填空。 | |
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| 1. Outgoing people arc easy to _ . 2. I'm afraid of speaking many people. 3. If you can't sleep at night, why not _ before going co bed? 4. Every time we don't know what to do, my father will ___ some good ideas. 5. Nobody could take away the books from the . |