13. $\cos 20^{\circ} \cdot \cos 40^{\circ} \cdot \cos 60^{\circ} \cdot \cos 80^{\circ}=$
$\begin{array}{l}\text { 13. } \frac{1}{16} \cdot \\ \cos 20^{\circ} \cdot \cos 40^{\circ} \cdot \cos 60^{\circ} \cdot \cos 80^{\circ} \\ =\frac{2 \sin 20^{\circ} \cdot \cos 20^{\circ} \cdot \cos 40^{\circ} \cdot \cos 60^{\circ} \cdot \cos 80^{\circ}}{2 \sin 20^{\circ}} \\ =\frac{2 \sin 40^{\circ} \cdot \cos 40^{\circ} \cdot \cos 60^{\circ} \cdot \cos 80^{\circ}}{4 \sin 20^{\circ}} \\ =\frac{2 \sin 80^{\circ} \cdot \cos 80^{\circ} \cdot \cos 60^{\circ}}{8 \sin 20^{\circ}}=\frac{\sin 160^{\circ}}{16 \sin 20^{\circ}}=\frac{1}{16} .\end{array}$