拋物線
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716671545.png)
的焦點(diǎn)為
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716687302.png)
,點(diǎn)
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716703528.png)
為該拋物線上的動(dòng)點(diǎn),,又已知點(diǎn)
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716718569.png)
,則
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716734563.png)
的取值范圍是
.
試題分析:由拋物線的定義可得
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716765598.png)
,又
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/201408240317167811166.png)
,
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/201408240317168121710.png)
,
當(dāng)
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716827367.png)
時(shí),
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716843617.png)
;當(dāng)
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716859400.png)
時(shí),
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/201408240317168901701.png)
,
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716905235.png)
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716921789.png)
,當(dāng)且僅當(dāng)
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716937427.png)
即
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716952383.png)
時(shí)取等號(hào),于是
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716968639.png)
,
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716983195.png)
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716999711.png)
,
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716983195.png)
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/201408240317170301054.png)
,
綜上所述
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716734563.png)
的取值范圍是
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824031716749422.png)
.
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