A line in the plane of a triangle is called an \emph{equalizer} if it divides into two regions having equal area and equal perimeter. Find positive integers , with as small as possible, such that there exists a triangle with side lengths that has exactly two distinct equalizers.
Solution
The desired integers are . Suppose we have a triangle with , , and . Say that a line is an \textit{area equalizer} if it divides into two regions of equal area. A line intersecting must intersect two of the three sides of . First consider a line intersecting the segments at and at , and let , . This line is an area equalizer if and only if , that is, . Since and , the area equalizers correspond to values of with and . Such an area equalizer is also an equalizer if and only if , where is the perimeter of . If we write , then we want to solve for . Now note that is convex, , and ; it follows that there is exactly one solution to in . Similarly, for equalizers intersecting on the sides and , we want to solve where and ; since is convex and , , there are no such solutions. It follows that if has exactly two equalizers, then it must have exactly one equalizer intersecting on the sides and . Here we want to solve where and . Now is convex and , ; thus has exactly one solution if and only if there is with and . The first condition implies , and then the second condition gives . Note that is in since and . We conclude that has two equalizers if and only if . Note that works. We claim that this is the only possibility when are integers and . Indeed, the only integers such that and is a perfect square are , , , , and , and the first four possibilities do not produce triangles since they do not satisfy . This gives the claimed result.