Pi (3.14159...) seems to get all of the attention in mathematics. On some level this is warranted. The ratio of the circumference of a circle to the diameter. Seems pretty pure. But what about the ratio of the circumference to the radius (which is two times pi and referred to as "tau")? Now that you know a bit of trigonometry, you'll discover in videos made by Sal and Vi that "tau" may be much more deserving of the throne!
Please excuse the simple math and spelling errors. You shouldn't believe anything I say without double-checking even at the best of times.
Go here: http://tauday.com/
and here: http://www.math.utah.edu/~palais/pi.html