I came upon an article on <a href="http://www.guardian.co.uk/life/science/story/0,12996,1320625,00.html">the greatest equations of all time</a>. I was inspired. So, along similar lines as the coolest number thread, what is your favourite formula (from math or science), equation, theorem, etc.?
Mine is the Binomial Theorem.
I always thought it was very elegant and simple, but powerful.