## Double & Half Angle Trig Identities

https://www.youtube.com/watch?v=PbrJHqBrSTk For sine and cosine, there are double and half angle trigonometric identities which we explore here. In particular, the cosine double angle identity has three forms, from among which we can choose one strategically depending on the situation. We prove these identities using the addition identities and the main Pythagorean identity.