Trig Double Angle Formula

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Taught by YourMathGal
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Lesson Summary:

In this math video, Julie Harland teaches the derivation of double angle trigonometry formula. She demonstrates examples using the formulas for cosine, sine, and tangent. There are three different ways to compute the cosine of 2A, two ways to compute the sine of 2A, and one way to compute the tangent of 2A. Julie also provides tips on how to choose which formula to use based on the information provided. Additionally, she shows how the tangent of 2A can be calculated using the equations for cosine and sine.

Lesson Description:

Video derives the double angle trigonometry formula and provides examples using the formula.

More free YouTube videos by Julie Harland are organized at

Questions answered by this video:
  • What are the trig double angle formulas?
  • What are the double angle identities for sine, cosine, and tangent?
  • How do you derive the double angle identities for sine, cosine, and tangent?
  • How do you derive cos(2A)?
  • How do you derive sin(2A)?
  • How do you derive tan(2A)?
  • Why are there different double angle formulas and how do you derive them?
  • Why can cos(2A) equal cos^2(A) - sin^2(A), 1 - 2sin^2(A), or 2cos^2(A) - 1?
  • Staff Review

    • Currently 4.0/5 Stars.
    This lesson will show you how to derive the formulas for sin(2A), cos(2A), and tan(2A) using the formulas that you should already know from the previous lessons in this series. All steps involved in the derivation of these double angle formulas are shown and explained.