Example 6, Fold a rectangle $A B C D$ with length and width of 4 and 3 respectively along the diagonal $A C$ to form a right dihedral angle, find the dihedral angle $D-A B-C$.
Let the dihedral angle $D-A B-C=a$, $\angle D A C=\theta, \angle D A B=\theta_{1}, \angle B A C=\theta_{2}$.
By the corollary of the three-line theorem,
$$
\begin{aligned}
\cos \angle D A B & =\cos \angle D A C \cos \angle B A C \\
& =\frac{3}{5} \times \frac{4}{5}=\frac{12}{25},
\end{aligned}
$$

i.e., $\cos \theta_{1}=\frac{12}{25}$.
By the three-line theorem,
$$
\begin{array}{l}
\cos \theta = \cos _{\theta_{1}} \cos \theta_{2}+\sin _{\theta_{1}} \sin \theta_{2} \cos \alpha \\
\frac{3}{5}=\frac{12}{25} \cdot \frac{4}{5}+\sqrt{\frac{481}{625}} \cdot \frac{3}{5} \cos \alpha \\
\cos \alpha=\frac{9}{\sqrt{481}} .
\end{array}
$$

Therefore, the dihedral angle $D-A B-C=\arccos \frac{9}{\sqrt{481}}$.
From this, it can be seen that for more complex problems of finding dihedral angles,

using the three-line theorem does not require adding auxiliary lines and can simplify the solution.