In this math lesson, Julie Harland derives the half-angle trig identities for cosine, sine, and tangent. She explains how to use double-angle identities for the cosine of 2x, and then goes on to derive the formulas for the half-angle identities. She shows three formulas for the tangent of a half-angle and explains that if you know the sine and cosine of an angle, you can use these formulas to figure out the sine, cosine, or tangent of half that angle.
Video derives the half angle trigonometry identities for cosine, sine and tangent
More free YouTube videos by Julie Harland are organized at http://yourmathgal.com