3. In $\triangle A B C$, $a, b, c$ are the sides opposite to $\angle A, \angle B, \angle C$ respectively. $a=15, b=17, \angle A=\theta(\theta$ is a constant). If the $\angle C$ of the triangle satisfying the above conditions is unique, then $\operatorname{tg} C=$ $\qquad$
$\angle A=g$ is a constant, $a<b, \angle A$ must be an acute angle. Given that $\angle C$ has a unique value satisfying the conditions of the problem, thus $\angle E$ $=180^{\circ}-\angle C-\theta$ is also unique, which means $\angle B=90^{\circ}$. Therefore, $c=\sqrt{17^{2}-15^{2}}=8, \cos C=\frac{8}{15}$.