$\left[\begin{array}{ll}{[\text { Auxiliary similar triangles }} \\ {[\quad \text { Similarity criteria }}\end{array}\right]$

On the side $A B$ of triangle $A B C$, a point $D$ is marked, and $\angle B C D=\angle A$. It is known that $B C=a, A C=b, A B=$ c. Find $C D$.
Triangles $C B D$ and $A B C$ are similar by two angles. Therefore, $C B: A C = B C: A B$. Hence, $C D = B C \cdot A C / A B$ $= a b / c$.

## Answer

$a b / c$.