a. Ta có: $SA\perp (ABC)\Rightarrow SA\perp AB, AC, BC\Rightarrow\Delta SAB, SAC\bot A$
$\left\{\begin{array}{I}BC\perp AB\text{ (}\Delta ABC\bot B\text{)}\\BC\bot SA\text{ (}SA\bot (ABC)\text{)}\end{array}\right.$
$\to BC\perp (SAB)\to BC\perp SB\to \Delta SBC\bot B$.
b. Ta có:
$\left\{\begin{array}{I} BC\perp AD\text{ (}BC\bot (SAB)\text{ chứng minh trên)}\\AD\bot SB\text{ (giả thiết)}\end{array}\right.$
$\to AD\bot(SBC)$
mà $DE\subset(SBC)\to AD\bot DE\to \Delta ADE\bot D$
Ta có:
$\left\{\begin{array}{I}AE\bot SC\text{ (giả thiết)}\\AD\bot SC\text{ (do }AD\bot(SBC)\text{ chứng minh trên)}\end{array}\right.$
$\to SC\bot(ADE)$
$\to SC\bot DE$.