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.
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