1 . 以下四个关于圆锥曲线的命题中:
①设A、B为两个定点,k为非零常数,若
,则动点P的轨迹为双曲线;
②抛物线
焦点坐标是
;
③过定圆C上一定点A作圆的动弦AB,O为坐标原点,若
,则动点P的轨迹为椭圆;
④曲线
与曲线
(
且
)有相同的焦点.
其中真命题的序号为______ (写出所有真命题的序号.)
①设A、B为两个定点,k为非零常数,若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2f4562ddd51f544eaff300c9b4638646.png)
②抛物线
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/315513892f06ef3ce82021d5e0566606.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/532eff5d4385426065732b3deec31426.png)
③过定圆C上一定点A作圆的动弦AB,O为坐标原点,若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/10a6bb839093992d42c6c813c73ccad2.png)
④曲线
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e9a0bd4bd9985f1e367c100b453ed03f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4f4e82ca0fbf98b8903c97467acd1d6a.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c69c2e4498e7916dbc05a71fc0c316ca.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/fe5c9d9221866bca2fd57b9b31100c62.png)
其中真命题的序号为
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