English: Two right triangles, one with side 7 and hypotenuse 13, and the other with side 13 and hypotenuse 17, have equal third sides of length √120. The square of this side, 120, is a congruum: it is the difference between consecutive values in the arithmetic progression of squares 7², 13², 17². Equivalently, the two annuli between the three yellow circles have equal areas, π times the congruum.