Let a point A lie in the interior of an acute angle. Find points B and C on the sides of the angle (one per side) such that the perimeter of ABC is minimal.
Let AB and AC be the reflections of A in the sides of the angle. Then the perimeter of ABC equals AB + BC + CA = ABB + BC + CAC. The latter is never shorter than the distance between AB and AC, so that
AB + BC + CA ABAC.
The equality is achieved when B and C are chosen to be the points of intersection of the corresponding sides of the angle with ABAC.