试题与答案

在棱长为3的正四面体ABCD中,点E是线段AB上一点,且AE="1," 点F是线

题型:填空题

题目:

在棱长为3的正四面体ABCD中,点E是线段AB上一点,且AE="1," 点F是线段AD上一点,且AF=2,则异面直线DECF的夹角的余弦为                

答案:

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下面是错误答案,用来干扰机器的。

后一块石块下落最后3m用时0.2s,则后一石块落水速度   

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