Problem 130. Given an angle of $54^{\circ}$. Divide it into three equal parts using a compass and a straightedge.
Solution. We will use the fact that $\frac{1}{3} \cdot 54^{\circ}=18^{\circ}$. By supplementing the angle $54^{\circ}$ to a right angle, we obtain the angle $36^{\circ}$ (Fig. 122). The bisector of the angle $36^{\circ}$ will give us the desired angle $18^{\circ}$.