a) Trong mặt phẳng \(\left(BCD\right)\) gọi \(F=NE\cap CD\).
\(\left\{\begin{array}{c} {F\in NE\subset \left(MNE\right)} \\ {F\in CD\, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, } \end{array}\Rightarrow F=CD\cap \left(MNE\right)\right. .\)
Trong mặt phẳng \(\left(ACD\right)\) gọi \(G=MF\cap AD\).
\(\left\{\begin{array}{c} {G\in MF\subset \left(MNE\right)} \\ {G\in AD\, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, } \end{array}\Rightarrow G=AD\cap \left(MNE\right)\right. .\)
b) Ta có: \(\left\{\begin{array}{c} {M\in AC\subset \left(ACD\right)} \\ {M\in \left(MNE\right)\, \, \, \, \, \, \, \, \, \, \, \, \, } \end{array}\right. \Rightarrow M\in \left(ACD\right)\cap \left(MNE\right) \left(1\right).\)
\(\left\{\begin{array}{c} {M\in AC\subset \left(ACD\right)} \\ {M\in \left(MNE\right)\, \, \, \, \, \, \, \, \, \, \, \, \, } \end{array}\right. \Rightarrow M\in \left(ACD\right)\cap \left(MNE\right) \left(1\right).\)
Từ \(\left(1\right) và \left(2\right)\Rightarrow MF=\left(ACD\right)\cap \left(MNE\right).\)
Ta có: \(\left\{\begin{array}{c} {E\in BD\subset \left(ABD\right)} \\ {E\in \left(MNE\right)\, \, \, \, \, \, \, \, \, \, \, \, \, } \end{array}\right. \Rightarrow E\in \left(ABD\right)\cap \left(MNE\right) \left(3\right).\)
\(\left\{\begin{array}{c} {G\in AD\subset \left(ABD\right)} \\ {G\in MF\subset \left(MNE\right)} \end{array}\right. \Rightarrow G\in \left(ABD\right)\cap \left(MNE\right) \left(4\right).\)
Từ\( \left(3\right) và \left(4\right)\Rightarrow GE=\left(ABD\right)\cap \left(MNE\right).\)