Trigonometry
Trigonometry (literally the measure "metry" of triangles "trigons") is an important topic. It seems that no matter where you go, triangles are to be found.
If you want to learn trigonometry for the first time, try this: http://www.clarku.edu/~djoyce/trig/
Identities
Given a right triangle with sides of lengths A, B, and C, and with angles opposite those sides of a, b, and c, and where c is a right angle:
<math>c^2 = a^2 + b^2 \,</math> (Pythagorean's Theorem)