8. In $\triangle A B C$, it is known that $\angle A=\alpha, C D 、 B E$ are the altitudes on $A B 、 A C$, respectively, then $\frac{D E}{B C}=$ $\qquad$ .
Given that points $B, D, E, C$ are concyclic $\Rightarrow \angle A E D=\angle A B C$
$$
\Rightarrow \triangle A E D \sim \triangle A B C \Rightarrow \frac{D E}{B C}=\frac{A D}{A C}=|\cos \alpha| \text {. }
$$