Here's a visualization of the angle addition formulae, [math]\sin(\alpha+\beta)=\sin \alpha \cos \beta + \cos \alpha \sin\beta [/math] and [math]\cos(\alpha+\beta)=\cos \alpha \cos \beta - \sin \alpha \sin\beta [/math], in the first quadrant of the unit circle. Start the slider at step 1, answer the questions as they appear, and you should be able to follow the argument. Prerequisite: basic understanding of right triangle trigonometry and unit circle trigonometry.
