Originating author is William Mc Callum.
If two triangles have the same area and the same perimeter, are they necessarily congruent? It turns out that the answer is no. For example, the triangle with sides and has the same area and perimeter as the triangle with sides , , and .
Both triangles have perimeter :
Amazingly, the two triangles also have the same area. The right triangle has area . To find the area of the other triangle, we use Heron’s formula, which states that the area of a triangle with side lengths , , and , is given by
where is the semiperimeter of the triangle. A quick calculation using this formula shows that the area of the second triangle is also 6.
The space of triangles
How do we find examples like this? The secret is to find the right way of representing the space of all triangles. There are many possible ways to do this. One way is to represent a triangle by the triple consisting of its three side lengths in some order. In this way we represent a triangle by a point in space. Not every point corresponds to a triangle; for example, all the coordinates must be positive. Can you think of other restrictions?
There’s another way of putting coordinates on the space of triangles using angles instead of lengths. Every triangle has an inscribed circle, and the radius of the circle has a simple relationship with the area and semiperimeter , namely
To see why this is true, drop perpendiculars from the center of the circle to the sides of the triangle, as in the left diagram in Figure 2. These perpendiculars form the altitudes of 3 smaller triangles with bases on the sides of the big triangle and vertices at the center of the inscribed circle. Adding up the areas of these triangles we get .
This equation tells us that if two triangles have the same area and same semiperimeter, then the radii of their inscribed circles are also the same. So if we are looking for two such triangles we will find them amongst all triangles inscribed around a fixed circle. Instead of using lengths to describe these triangles, we will use the angles formed by the three radii at the center of incircle, as in the right diagram in Figure 2.
Parameterizing triangles with constant area and perimeter
Inside the space of triangles we can find curves corresponding to a whole family of triangles with the same values of and .
First, we express in terms of the angles , , and and the radius of the incircle, as follows. The radii and the lines from the vertices to the incenter break the triangle into six right triangles. Because the lines from the vertices to the center bisect the angles of the big triangle, these right triangles occur in congruent pairs. Taking one base length from each pair and adding, we get
Second, we translate this condition into an equation defining a curve in the plane. Let , , and . Since , we have
Then, if is the constant , equation (1) becomes for fixed , the equation
Every triangle with area and semiperimeter determines a point on this curve, and every point on the curve in a certain region of the plane corresponds to a triangle. The region corresponds to angles that actually work in Figure 2, namely angles satisfying and , which corresponds to the region , and (since ).
The following figure shows this curve for , the value corresponding to the triangle with sides 3, 4, and 5. Every point on the component of this curve in the positive quadrant corresponds to a triangle; the side lengths of the triangle are , , and . In particular, the points , , , , , and all correspond to the triangle with sides , , and , with the sides taken in different orders. This figure is interactive: see what happens for some other points on the curve or some other values for the area and perimeter!
Finding points on the curve
Because the curve in Figure 3 is defined by an equation of degree 3, we can find points on it using the method of tangents and secants. Two points on the curve determine a secant which cuts the curve in one more point; finding the point amounts to solving a cubic equation in , two of whose roots are already known. Since we already have 6 points on the curve, there are lots of possibilities for secants, and generating more points generates more possibilities. In fact, the curve has infinitely many points with rational coordinates. The two-secant procedure illustrated in Figure 3 leads to the point (marked with a circle), which corresponds to the triangle with sides , , and .
The secant procedure works for any cubic curve in the plane; such curves are called elliptic curves (not because the curves are themselves ellipses, but because they arise in the study of a certain class of complex functions called elliptic functions). The secant procedure allows one to define a group structure on the set of rational points on a elliptic curves (that is, points whose coordinates are rational numbers).
The study of elliptic curves is a central area of research in number theory, with applications to the cryptographic schemes behind secure financial transactions on the web. Elliptic curves played a central role in the proof of Fermat’s Last Theorem.
The story described in this article shows the remarkable unity of mathematics, starting as it does in high school and ending in research. Along the way we encountered a fundamental idea in modern mathematics: the idea of solving a problem about a particular type of object (triangles with area 6 and perimeter 12, for example) by situating the object in a more general space (the space of all triangles) and finding the right way of parameterizing that space.