The Theorem of Pythagoras

The Theorem of Pythagoras is a specific case of the Cosine Law that applies specifically to right angle triangles. With it, and given any 2 sides of a right angle triangle, you can find the third side. Then having solved all the sides of the triangle, you can use the standard trig identities (Sine, Cosine, Tangent) to evaluate the size of all the angles.

Specifically, if we have a right angle triangle with sides a, b, and c, with c being the hypotenuse, we can write:

c^2 = a^2 + b^2

Or: "The square of the hypotenuse is the sum of the squares of the other two sides."

This is one of the most basic trig concepts, and is probably one of the first concepts that were taught in trig... very important and very useful!