The paper proves the following result: Consider a circle and a right triangle $ABC$ such that $AB$ is a diameter of the circle. Draw semicircles with diameters $AC$ and $BC$. Each of this semicircles, together with the original circle determines a lune. The sum of the areas of these lunes equals the area of the triangle $ABC$. The result is elementary and not very surprising, Nevertheless it can be a nice excercise for secondary school students and it is suitable to be worked out with GeoGebra.

Reviewer:

Antonio M. Oller (Zaragoza)