如图,菱形ABCD的边长为4,∠BAD=60°,AC∩BD=O,将菱形ABCD沿对角线AC折起,得到三棱锥B-ACD,点M是棱BC的重点,DM=2√2,求证二面角D-AB-O的余弦值
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![如图,菱形ABCD的边长为4,∠BAD=60°,AC∩BD=O,将菱形ABCD沿对角线AC折起,得到三棱锥B-ACD,点M是棱BC的重点,DM=2√2,求证二面角D-AB-O的余弦值](/uploads/image/z/10401444-36-4.jpg?t=%E5%A6%82%E5%9B%BE%2C%E8%8F%B1%E5%BD%A2ABCD%E7%9A%84%E8%BE%B9%E9%95%BF%E4%B8%BA4%2C%E2%88%A0BAD%3D60%C2%B0%2CAC%E2%88%A9BD%3DO%2C%E5%B0%86%E8%8F%B1%E5%BD%A2ABCD%E6%B2%BF%E5%AF%B9%E8%A7%92%E7%BA%BFAC%E6%8A%98%E8%B5%B7%2C%E5%BE%97%E5%88%B0%E4%B8%89%E6%A3%B1%E9%94%A5B-ACD%2C%E7%82%B9M%E6%98%AF%E6%A3%B1BC%E7%9A%84%E9%87%8D%E7%82%B9%2CDM%3D2%E2%88%9A2%2C%E6%B1%82%E8%AF%81%E4%BA%8C%E9%9D%A2%E8%A7%92D-AB-O%E7%9A%84%E4%BD%99%E5%BC%A6%E5%80%BC)
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