\(\left\{{}\begin{matrix}M\in AB\subset\left(ABN\right)\Rightarrow M\in\left(ABN\right)\\N\in\left(ABN\right)\end{matrix}\right.\) \(\Rightarrow MN\in\left(ABN\right)\)
\(\left\{{}\begin{matrix}M\in\left(SMC\right)\\N\in SC\subset\left(SMC\right)\end{matrix}\right.\) \(\Rightarrow MN\in\left(SMC\right)\)
\(\Rightarrow MN=\left(ABN\right)\cap\left(SMC\right)\)
b.
Trong mp (ABCD), nối DM kéo dài cắt BC kéo dài tại E
\(\left\{{}\begin{matrix}N\in\left(DMN\right)\\E\in DM\in\left(DMN\right)\end{matrix}\right.\) \(\Rightarrow NE\in\left(DMN\right)\)
\(\left\{{}\begin{matrix}N\in SC\in\left(SBC\right)\\E\in BC\in\left(SBC\right)\end{matrix}\right.\) \(\Rightarrow NE\in\left(SBC\right)\)
\(\Rightarrow NE=\left(DMN\right)\cap\left(SBC\right)\)
