Ta có:
$SA\perp (ABCD)$
$CD\subset (ABCD)$
$\to SA\perp CD$
mà $CD\perp AD$ (hình vuông)
nên $CD\perp (SAD)$
Lại có: $AN\subset (SAD)$
$\to CD\perp AN$
mà $AN\perp SD$
nên $AN\perp (SCD)$
Do $SC\subset (SCD)$
nên $AN\perp SC$
Ta có:
$\quad \left.\begin{array}{l}SC\perp AM\quad (\rm câu \,\,a)\\SC\perp A\quad (cmt)\\AM\subset (AMN)\\AN\subset (AMN)\end{array}\right\}\longrightarrow SC\perp (AMN)$
