a,Goi $E=MN\cap AD\Rightarrow E=(\alpha )\cap AD$
$F=MN\cap CD\Rightarrow F=(\alpha)\cap CD$
$L=SD\cap FP\Rightarrow L=(\alpha) \cap SD$
b,
$\left.\begin{matrix}I=MQ \in NP \Rightarrow I\in (SAB)\cap (SBC)\\ SB=(SAB)\cap(SBC)\end{matrix}\right\}$$\Rightarrow I\in SB$hay $I,S,B$ thang hang
c,
thiet dien cua hinh chop khi cat boi mp $(\alpha)$ la da giac $MNPLQ$
d,
$\left.\begin{matrix}Goi O=AN\cap CM\Rightarrow O\in(SAN)\cap(SCM)\\ S\in(SAM)\cap(SCN)\end{matrix}\right\}$$\Rightarrow SO=(SAN)\cap(SCM) (1)$
Goi $J=QN\cap PM$
$\left.\begin{matrix} J\in QN\subset (SAN)\\J\in PM\subset (SCM) \end{matrix}\right\}$$\Rightarrow J\in (SAN)\cap (SCM) (2)$
$ (1),(2)\Rightarrow j\in SO$ co dinh