过点C(0，1)的椭圆
的离心率为
椭圆与x轴交于两点A(a，0)、B(-a，0)，过点C的直线l与椭圆交于另一点D，并与x轴交于点P，直线AC与直线BD交于点Q。
当直线l过椭圆右焦点时，求线段CD的长；
参考答案：
由已知得b=1，
，解得a=2，所以椭圆方程为
椭圆的右焦点为
此时直线l的方程为
，代入椭圆方程化简得
解得
代入直线l的方程得
，所以
故
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