Pythagoras' Theorem and pizzas? The theorem is meticulously described here with an animation and extension material that brings the concept to real life (yes, using pizzas!). Among various other sections covered are sine, cosine, tangent and measurements in radians.
The double-angle formulae
Double angle formulae are so called because they involve trigonometric functions of double angles e.g. sin 2A, cos 2A and tan 2A. Video tutorial 26 mins.