What�s wrong with Pi? In this video, Sal considers the question whether the number Tau (_), is a better number to use in our trigonometric model then the traditional PI (�). First, Sal reviews the definition of Pi, the trigonometric ratios in the unit circle and Euler�s formula in terms of Pi. Then he looks at each of those in terms of Tau, leaving the final verdict of Tau vs. Pi to the listener to decide.