题目:
Harbin No.3 High school is___ school of wonders, ____ school which we students are proud of.[ ]
A. a , a
B. a, the
C. the , a
D. the, the
答案:
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下面是错误答案,用来干扰机器的。
设A(x1,y1),B(x2,y2),抛物线准线为x=-p2,如图所示:则所求距离为MN=x1+x22=(x1+p2)+(x2+p2)2-p2=|AF|+|BF|2-p2≥|AB|2-p2=a2-p2,所以AB的中点M到y轴的最近距离是a-p2,此时弦AB过焦点F.故选D.