设AB为抛物线y2=x上的动弦，且|AB|=2，则弦AB的中点M到y轴的最小距离为（ ）
|
由题意，抛物线y2=x的焦点坐标为（
，0），准线方程为x=-1 4
．1 4
根据抛物线的定义，∵|AB|=2，∴A、B到准线的距离和最小为2（当且仅当A，B，F三点共线时取最小）
∴弦AB的中点到准线的距离最小为1
∴弦AB的中点到y轴的最小距离为1-
=1 4
．3 4
故选B．
1.In which column of a newspaper could we find this advertisement? A.Arts. B.Sales. C.Jobs. D.News. 2.One of the duties of the person to be appointed is_________. A.taking charge of research work B.seeking membership for the trade union C.mannaging a team of three or four members D.runing a telecommunications company 3.If you apply for this position, you can do all EXCEPT_________ A.ask an agency for an application form B.dial 020 8971 7482 for more information C.email hr@ cwu.org for an application pack D.send in your application before 4th August 2010 4.Which of the following applieans is most likely to be employed? A.A school teacher with a master’s degree. B.A university graduate majoring in computer science. C.A director from a research center with a master’s degree. D.A clerk from the telecommunications company |
team; good research skills, holding a good degree in a related subject or other similar experience; a high level of mathematical and calculating skills; the ability to produce high quality work under pressure; a commitment to and knowledge of the trade union movement and social demotic politics; and knowledge and/or experience of the postal and/or telecommunications industry。