a.
\(\left\{{}\begin{matrix}SA\perp\left(ABCD\right)\Rightarrow SA\perp BC\\BC\perp AB\left(gt\right)\end{matrix}\right.\)
\(\Rightarrow BC\perp\left(SAB\right)\)
b.
\(\left\{{}\begin{matrix}SA\perp\left(ABCD\right)\Rightarrow SA\perp BD\\BD\perp AC\left(\text{hai đường chéo hình vuông}\right)\end{matrix}\right.\)
\(\Rightarrow BD\perp\left(SAC\right)\)
c.
Theo câu a, do \(\left\{{}\begin{matrix}BC\perp\left(SAB\right)\\AH\in\left(SAB\right)\end{matrix}\right.\) \(\Rightarrow BC\perp AH\)
Lại có \(AH\perp SB\left(gt\right)\)
\(\Rightarrow AH\perp\left(SBC\right)\)