a) Trong mặt phẳng \(\left(ABC\right)\), gọi \(D=EF\cap BC.\)
Ta có \(\left\{\begin{array}{l} {D\in BC\subset \left(SBC\right)} \\ {D\in EF\subset \left(MEF\right)} \end{array}\right. \Rightarrow D\in \left(SBC\right)\cap \left(MEF\right)\, \, \, \, \left(1\right).\)
\(M\in \left(SBC\right)\cap \left(MEF\right)\, \, \, \, \left(2\right).\)
Từ \(\left(1\right) \)và \(\left(2\right)\) suy ra \(MD=\left(SBC\right)\cap \left(MEF\right).\)
b) Trong mặt phẳng \(\left(SBC\right)\), gọi \(J=MD\cap SB.\)
Trong mặt phẳng \(\left(MEF\right)\), gọi \(I=EJ\cap MF\).
Ta có \(\left\{\begin{array}{l} {H\in SA\subset \left(SAB\right)} \\ {H\in NF\subset \left(MNF\right)} \end{array}\right. \Rightarrow H\in \left(SAB\right)\cap \left(MNF\right)\, \, \, \, \left(3\right).\)
\(\left\{\begin{array}{l} {I\in EJ\subset \left(SAB\right)} \\ {I\in MF\subset \left(MNF\right)} \end{array}\right. \Rightarrow I\in \left(SAB\right)\cap \left(MNF\right)\, \, \, \, \left(4\right).\)
Từ \(\left(3\right)\) và \(\left(4\right)\) suy ra \(HI=\left(SAB\right)\cap \left(MNF\right).\)