18. Construct a triangle of the smallest perimeter, two vertices of which lie on the sides of a given angle, and the third one at a given point inside the angle.
18. Let $A_{1}$ and $A_{2}$ be the points symmetric to point $A$ with respect to the sides of angle MON. The points of intersection of segment $A_{1} A_{2}$ with the sides of the angle give the vertices of the desired triangle. $A_{1} C=C A, D A=D A_{2}, A_{1} A_{2}$ is a straight line.

$2 p=A C+C D+D A=A_{1} C+C D+D A_{2}$ (smallest).