How to Multiply Numbers by 9 Using Nothing but Your Fingers

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. With this free math tutorial, you'll learn how to calculate the area of a triangle by formula. In this clip, you'll learn an easy finger-counting trick for quickly calculating multiples of 9.