a) Ta có:
$(SAC) \cap (SBD) = \left\{S\right\}$
Trong $mp(ABCD)$ gọi $AC\cap BD = \left\{E\right\}$
$E\in AC; \, AC\subset (SAC) \Rightarrow E \in (SAC)$
$E\in BD;\, BD \subset (SBD) \Rightarrow E \in (SBD)$
$\Rightarrow (SAC) \cap (SBD) = \left\{E\right\}$
$\Rightarrow (SAC) \cap (SBD) =SE$
b) Ta có:
$(SAB) \cap (SCD) = \left\{S\right\}$
Trong $mp(ABCD)$ gọi $AB\cap CD = \left\{F\right\}$
$F\in AB; \, AB\subset (SAB) \Rightarrow F \in (SAB)$
$F\in CD;\, CD \subset (SCD) \Rightarrow F \in (SCD)$
$\Rightarrow (SAB) \cap (SCD) = \left\{F\right\}$
$\Rightarrow (SAB) \cap (SCD) = SF$
