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问题 填空题
若圆x2+y2+mx-
1
4
=0与直线y=-1相切，且其圆心在y轴的左侧，则m的值为______
答案

圆方程配方得（x+

m
2
）2+y2=
m2+1
4
，圆心为（-
m
2
，0）．

由条件知-

m
2
＜0，即m＞0．

又圆与直线y=-1相切，则0-（-1）=

m2+1
4
，即m2=3，∴m=
3
．

完形填空

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单项选择题

170在电话网中的服务业务是（）。

A.号码查询台

B.集中付费业务

C.自动话费查询台

D.报时台
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