I know another way which is basically the same and the why. So it is just base x height divided by 2. The base of a square for example can be 4. Then the height of the square (the length) can be 3. Cutting a square in several pieces it is possible to get a triangle. Triangles are contained in any polygon.

@nastya So this is a triangle that is scaled to a bigger end. So we can start with the small one and just scale it.