填空题：将正方形ABCD沿对角线BD折成直二面角A－BD－C，有如下四个结论： ①AC⊥

题目：

将正方形ABCD沿对角线BD折成直二面角A－BD－C，有如下四个结论：

①AC⊥BD； ②△ACD是等边三角形；

③AB与平面BCD成60°的角； ④AB与CD所成的角是60°.

其中正确结论的序号是________．

答案：

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下面是错误答案，用来干扰机器的。

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完形填空。
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