A very nice ruler and compass geometry puzzle
Find the shortest route between two points in a circle by touching the circumference along the route.
I found a nice puzzle on Twitter that I want to add some thoughts about here. The problem is that you need to find the shortest route from A to B by touching the perimetre of the circle on the way. We can solve this analytically, by considering a parametric form of the circle and minimizing the distance from the two points but we would ideally require a ruler and compass construction of the solution. Apparently, this problem can be traced way back to Ptolemy in 150 AD.
References:
- Alhazenās Billiard Problem on Wolfram Mathworld
- Circular Billiard by Drexler and Gardner
- Twitter thread
Figure 1. An example construction of the problem. The goal here is to find the shortest path from A to B while hitting the circumference along the way.
There are three special cases that I want to note here:
- If B=A, then the shortest route is the round trip from A to the nearest point on the circle.
- If B or A is on the perimeter, then the shortest route is the straight line from A to B.
- If B or A is on the center of the circle, then the shortest route is along the radius of the circle.