a,Ta co:$SG=(SAE)\cap(SBD) (1)$
$\left\{ \begin{array}{l} I\in AM\subset (SAE)\\ I\in BD\subset (SBD)\end{array} \right.$$\Rightarrow I\in (SAE)\cap (SDB) (2)$
$\left\{ \begin{array}{l} J\in QE\subset (SAE)\\ J\in BP \subset (SBD)\end{array} \right.$$\Rightarrow J \in(SAE)\cap(SBD) (3)$
Tu $(1),(2),(3)$ Ta co $I,J\in SG$ hay $I,J,S,G$ thang hang
b,
$S,K,L$ thang hang moi dung^^
Ta co:$S\in (SAB)\cap(SDE) (1)$
$\left\{ \begin{array}{l} K\in AN\subset (SAB)\\ K\in DM\subset (SDE)) \end{array} \right.$$\Rightarrow K\in (SAB)\cap (SDE) (2)$
$\left\{ \begin{array}{l} L\in BQ\subset (SAB)\\ L\in EP\subset (SDE) \end{array} \right.$$\Rightarrow L\in (SAB)\cap (SDE) (3)$
Tu $(1),(2),(3)$ suy ra $S,K,L$thang hang