Vì a // (P) nên a // b sao cho b \( \subset \) (P)
\( \Rightarrow \) (\(\Delta \); a) = (\(\Delta \); b)
Mà \(\Delta \) \( \bot \) (P); b \( \subset \) (P) nên \(\Delta \) \( \bot \) b \( \Rightarrow \) (\(\Delta \); b) = 900
Vậy (\(\Delta \); a) = 900