cho hình chóp tam giác đều S.ABC . có cạnh đáy bằng 2a và cạnh bên bằng a. cosin góc giữa cạnh bên và mặt đáy bằng bao nhiêu
hình chóp ta giác đều có tâm là trọng tâm tam giác ABC => SG vuông góc với (ABC), giờ chỉ cần tìm 2/3AG rồi tìm cos trong tam giác SGA vuông tại G –  phamthuhong131 19-03-18 07:52 PM
Gọi O là trọng tậm tg ABC tính đk AO=$\frac{2a}{\sqrt{3}}$
$\Rightarrow cos=\frac{AO}{SA}=2/\sqrt{3}$

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