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 <a href="https://en.wikipedia.org/wiki/Congruum" class="extiw" title="en:Congruum">congruum</a>: it is the difference between consecutive values in the <a href="https://en.wikipedia.org/wiki/Arithmetic_progression" class="extiw" title="en:Arithmetic progression">arithmetic progression</a> of squares 7<sup>2</sup>, 13<sup>2</sup>, 17<sup>2</sup>. Equivalently, the two <a href="https://en.wikipedia.org/wiki/Annulus_(mathematics)" class="extiw" title="en:Annulus (mathematics)">annuli</a> between the three yellow circles have equal areas, π times the congruum.
<a href="//commons.wikimedia.org/wiki/User:David_Eppstein" class="mw-redirect" title="User:David Eppstein">David Eppstein</a>
<span class="int-own-work" lang="en">Own work</span>