How To: Define trigonometric functions with a unit circle

Learn how to define the three fundamental trigonometric functions using a unit circle. From Ramanujan to calculus co-creator Gottfried Leibniz, many of the world's best and brightest mathematical minds have belonged to autodidacts. And, thanks to the Internet, it's easier than ever to follow in their footsteps (or just finish your homework or study for that next big test). With this installment from Internet pedagogical superstar Salman Khan's series of free math tutorials, you'll learn how to define the sine, cosine and tangent functions within a unit circle.