6) Ta có:
$(SAC)\cap (MBD) = \left\{M\right\}$
Trong mp $(ABCD)$, gọi $AC\cap BD = \left\{N\right\}$
$N\in AC;\, AC \subset (SAC) \Rightarrow N \in (SAC)$
$N \in BD;\, BD \subset (MBD) \Rightarrow N \in (MBD)$
$\Rightarrow (SAC) \cap (MBD) = \left\{N\right\}$
$\Rightarrow (SAC)\cap (MBD) = MN$
7a) Ta có:
$(SAB) \cap (MCD) = \left\{M\right\}$
Trong mp $(ABCD)$, gọi $CD\cap AB = \left\{N\right\}$
$N\in AB; \, AB\subset (SAB) \Rightarrow N \in (SAB)$
$N\in CD;\, CD\subset (MCD) \Rightarrow N \in (MCD)$
$\Rightarrow (SAB) \cap (MCD) = \left\{N\right\}$
$\Rightarrow (SAB) \cap (MCD) = MN$
b) Ta có:
$(SAD) \cap (MBC) = \left\{M\right\}$
Trong mp $(ABCD)$, gọi $AD \cap BC = \left\{K\right\}$
$K\in AD;\, AD \subset (SAD) \Rightarrow K \in (SAD)$
$K \in BC;\, BC \subset (MBC) \Rightarrow K \in (MBC)$
$\Rightarrow (SAD) \cap (MBC) = \left\{K\right\}$
$\Rightarrow (SAD) \cap (MBC) = MK$
