Ta co:$K\in AD\Rightarrow K\in (ADG)\Rightarrow KG\in (ADG)$Trong mp $(SCD):L=DG\cap SC\Rightarrow L\in (SAC)\cap(ADG)$
Ma$A\in (SAC)\cap(ADG)$ nen $AL=(SAC)\cap(ADG)$
Trong mp $(ADG):I=AL\in KG\Rightarrow \left\{ \begin{array}{l} I\in KG\\ I\in AL\subset (SAC)\end{array} \right.$
Vay $I=KG\cap (SAC)$