Double-Angle, Half-Angle, and Sum/Difference Formulas

So many formulas, so little time! Luckily you'll never see most of these again - even in Calculus - so the key is to figure out exactly which ones your teacher expects you to have memorized, and go from there. In the videos below, I take you through the most important ones, and show you how they're used in typical homework and test questions.

Double-Angle Formulas:
sin2X = 2sinXcosX &
cos2X=cos2X-sin2X

Of all the formulas in the Trig Identities chapter, the double-angle formulas are the only ones you'll ever see again in Calculus. In this video we'll take a look at the double-angle formulas for sine and cosine and work a few examples. And I throw a proof in there, just in case you're in honors and have an aggro teacher.

Half-Angle Formulas for Sine, Cosine & Tangent

"Half-angle formula" and "double-angle formula" sound pretty similar, so you'd think they'd be equally important. Nope! But I give the half-angle formulas their own video anyway because they seem to generate the most confusion vis-a-vis which angle to pick for θ and θ/2.

Sum & Difference Formulas, and Even-Odd Properties

This video contains the remainder of the "popular" sum and difference formulas that almost every trig teacher seems to go over for sine, cosine & tangent. If you have to memorize these, I tell you tips for that as well, but most teachers will give them to you. Also, what do "even" and "odd" have to do with trig?

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