zhengang1981
春芽
共回答了18个问题采纳率:88.9% 举报
(1)
![](https://img.yulucn.com/upload/d/cc/dcce8b54ed4042fa3417c79399259db2_thumb.jpg)
, (2)
试题分析:(1)设椭圆的焦距为2 c ,因为
![](https://img.yulucn.com/upload/2/36/236b9a60eb780631bfb2363fb3907a35_thumb.jpg)
,所以有
![](https://img.yulucn.com/upload/f/83/f83e088e8b3eb03f508d43756c30635f_thumb.jpg)
,故有
![](https://img.yulucn.com/upload/e/43/e43262b0d37c93ce593440e84e998a58_thumb.jpg)
。从而椭圆C的方程可化为:
![](https://img.yulucn.com/upload/4/5e/45e93cb5d8f8c1441d73f1577cbe0b94_thumb.jpg)
①…………2分
易知右焦点 F 的坐标为(
![](https://img.yulucn.com/upload/c/04/c043f4696503dc892022a11a7e20477a_thumb.jpg)
),
据题意有 AB 所在的直线方程为:
![](https://img.yulucn.com/upload/f/11/f118b73e05c3a85987478532f7f31bfe_thumb.jpg)
②…………4分
由①,②有:
![](https://img.yulucn.com/upload/7/6c/76c9fe9127d19b108b936747dbcfc440_thumb.jpg)
③
设
![](https://img.yulucn.com/upload/2/50/250cabc0a5a0d24dd93402a810f2adce_thumb.jpg)
,弦 AB 的中点
![](https://img.yulucn.com/upload/5/a1/5a1b205ea0e99f30b2cda30b4417b86c_thumb.jpg)
,由③及韦达定理有:
所以
![](https://img.yulucn.com/upload/e/eb/eebe7e6a330d21ffee02e907d82a14f5_thumb.jpg)
,即为所求。…………6分
(2)设
![](https://img.yulucn.com/upload/f/43/f43b15e617cf5361b40d176caefebedc_thumb.jpg)
,由1)中各点的坐标有:
![](https://img.yulucn.com/upload/c/57/c57f66bd7c559635d02b248d95d83529_thumb.jpg)
,所以
![](https://img.yulucn.com/upload/2/34/23464809c44ffa0e0432d2a0a36aba62_thumb.jpg)
。
又点在椭圆C上,所以有
![](https://img.yulucn.com/upload/c/4e/c4e9258de5d293242f77423215b24e08_thumb.jpg)
整理为
![](https://img.yulucn.com/upload/e/90/e9028ac8cc91d20e62792a36dd202fc7_thumb.jpg)
。④………8分
由③有:
![](https://img.yulucn.com/upload/1/3a/13a717dfa465b13c41a78c80e9ff4163_thumb.jpg)
。
![](https://img.yulucn.com/upload/b/32/b32af42f2949448d44d905d994554a24_thumb.jpg)
⑤
又A﹑B在椭圆上,故有
![](https://img.yulucn.com/upload/0/a8/0a897de1ddce01b17706403d398907e1_thumb.jpg)
⑥
将⑤,⑥代入④可得:
![](https://img.yulucn.com/upload/3/b0/3b0b767c0f0c4e5861db6b8e0634ef97_thumb.jpg)
。…………10分
![](https://img.yulucn.com/upload/9/59/9595eff6160b3431dd618c40f0bd36f5_thumb.jpg)
,故有
所以
![](https://img.yulucn.com/upload/b/7d/b7d6e4d8217530340bc340bd2cc46fd8_thumb.jpg)
,
![](https://img.yulucn.com/upload/9/37/9378691111505fdc31e16632fb0e998a_thumb.jpg)
…………12分
点评:圆锥曲线的问题一般来说计算量大,对运算能力要求很高,寻求简洁、合理的运算途径很重要,在解答时注意以下的转化:⑴若直线与圆锥曲线有两个交点,对待交点坐标是“设而不求”的原则,要注意应用韦达定理处理这类问题 ; ⑵与弦的重点有关问题求解常用方法一韦达定理法 二 点差法;⑶平面向量与解析几何综合题,遵循的是平面向量坐标化,应用的是平面向量坐标运算法则还有两向量平行、垂直来解决问题,这就要求同学们在基本概念、基本方法、基本能力上下功夫.
1年前
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