[tex]\it I)\\ \\ \left.\begin{aligned}D'D\perp (ABC)\ \ \ \ \ \ \ \ \\ \\ DO\perp AC\ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ \\ DO,\ AC\ \subset\ (ABC) \end{aligned}\right\} \stackrel{T.3\perp}{\Longrightarrow}\ D'O\perp AC[/tex]
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[tex]\it II)\ \ D'D\perp (ABC)\ \d si\ AC\subset (ABC) \Rightarrow D'D\perp AC \Rightarrow AC\perp D'D\ \ \ \ (1)\\ \\ AC\perp OD\ \ \ \ \ (2)\\ \\ D'D,\ OD\ \subset\ (D'DO)\ \ \ \ \ (3)\\ \\ \\ (1),\ (2),\ (3) \Rightarrow AC\perp (D'DO)\ \ \ \ \ (4)\\ \\ D'D\subset\ (D'DO)\ \ \ \ (5)\\ \\ (4),\ (5) \Rightarrow AC\perp D'D \Rightarrow D'D\perp AC[/tex]
