a) Ta có:
$(SAN)\cap (SCM)=\left\{S\right\}$
Trong $mp(ABC)$ gọi $AN\cap CM =\left\{D\right\}$
$D\in AN;\, AN\subset (SAN)\Rightarrow D\in (SAN)$
$D\in CM;\, CM\subset (SCM)\Rightarrow D\in (SCM)$
$\Rightarrow (SAN)\cap (SCM)=\left\{D\right\}$
$\Rightarrow (SAN)\cap (SCM)= SD$
b) Ta có:
Trong $mp(ABC),\, KN\cap AB =\left\{A\right\}$
$A\in KN;\, KN\subset (IKN)\Rightarrow A\in (IKN)$
$A\in AB;\, AB\subset (SAB)\Rightarrow A\in (SAB)$
$\Rightarrow (SAB)\cap (IKN)=\left\{A\right\}$
Trong $mp(SBC)$, gọi $SB\cap IN = \left\{E\right\}$
$E\in IN;\, IN\subset (IKN)\Rightarrow E\in (IKN)$
$E\in SB;\, SB\subset (SAB)\Rightarrow E\in (SAB)$
$\Rightarrow (SAB)\cap (IKN)=\left\{E\right\}$
$\Rightarrow (SAB)\cap (IKN)=AE$
