[url=http://artofproblemsolving.com/wiki/index.php?title=1950_AHSME_Problems/Problem_48]http://artofproblemsolving.com/wiki/index.php?title=1950_AHSME_Problems/Problem_48[/url] gives a problem needing [url=https://en.wikipedia.org/wiki/Viviani%27s_theorem]Vivani's Theorem[/url].