Trong mp (ABCD) từ A kẻ \(AE\perp BD\), trong mp (SAE) từ A kẻ \(AF\perp SE\) (1)
Ta có: \(\left\{{}\begin{matrix}SA\perp\left(ABCD\right)\Rightarrow SA\perp BD\\BD\perp AE\end{matrix}\right.\) \(\Rightarrow BD\perp\left(SAE\right)\)
\(\Rightarrow BD\perp AF\) (2)
(1);(2) \(\Rightarrow AF\perp\left(SBD\right)\Rightarrow AF=d\left(A;\left(SBD\right)\right)\)
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