Complex Analysis, Video #15 (Complex Arithmetic, Part 15). Complex Multiplication in Polar Form. De Moivre's Formula. Application to Derivation of Trigonometric Identity.
Details: Review polar form, focusing especially on the exponential form. State De Moivre's formula. Apply De Moivre's formula to derive a couple trigonometric identities. Check answers with "TrigExpand" on Mathematica.
Mistake at the end: I forgot to get rid of the "i" in the identity for sin(3*theta).