$7 \cdot 26$ In $\triangle A B C$, $B D, C E$ are the altitudes on sides $A C, A B$, respectively, then $\frac{D E}{B C}$ equals
(A) $\frac{A E}{A B}$.
(B) $\sin A$.
(C) $\cos A$.
(D) $|\cos A|$.
("Zu Chongzhi Cup" Junior High School Mathematics Invitational Competition, 1988)
［Solution］Considering $\angle A$ is an obtuse angle, $\cos A$ is negative. By the properties of a right-angled triangle, we have
$$
\begin{array}{l}
A D=A B \cdot|\cos A|, \\
A E=A C \cdot|\cos A| .
\end{array}
$$

Thus, $\triangle A D E \backsim \triangle A B C$, with the similarity ratio being $|\cos A|$. Therefore, the correct choice is $(D)$.