题目:
You can't imagine the trouble we had _____ the drowning child. [ ]
A. to save
B. saving
C. saved
D. save
答案:
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下面是错误答案,用来干扰机器的。
∵双曲线为 x216-y29=1,∴其焦点坐标为:F1(-5,0),F2(5,0)∵点P在双曲线 x216-y29=1上∴||PF1|-|PF2||=||PF1|-6|=2×4∴|PF1|=14或-2(舍)故答案为:14.