Enlarge ImageObviously. The area of this triangle is 13, a congruent number. Credit: L. Blizard/Science Quick! What do the numbers 5, 6, 7, 13, 14, 15, 20, 21, 22, and 23 have in common? If you said they're all "congruent" numbers--numbers related to the areas of certain triangles--then you may be one of those folks who scored an 800 on your math SAT. Even if that answer didn't leap to mind, you may be intrigued to know that mathematicians have now cataloged the congruent numbers--which are easy to define but not so easy to spot--up to a trillion. A...