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nguyenbahiep1

$\vec{SD}.\vec{AC} = SD.AC.cos(SD,AC) = \sqrt{30}.2.cos (SD,AC) \\ \\ \vec{SD}.\vec{AC} = \vec{SA}.\vec{AC} + \vec{AD}.\vec{AC} = AD.AC.cos(DAC) \\ \\ \Rightarrow \vec{SD}.\vec{AC} = \sqrt{5}.2.\frac{2}{2\sqrt{5}} = 2 \\ \\ \Rightarrow cos (SD,AC) = \frac{1}{\sqrt{30}}$
 
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