Double angle formulas are called “double” angle because they involve trigonometric functions of double angles like sin 2x or cos 2x. In other words, when we use the formula, we’re doubling the angle. These formulas allow you to rewrite a double angle expression like sin 2x and cos 2θ into an expression with a single angle. This means that instead of working with a complicated expression, you can transform it to a simpler one. As well as their ability to help us simplify and solve equations, they can verify identities and are needed for some aspects of calculus involving analytic geometry .
List of Double Angle Formulas
The double angle formula for sin is:
The cosine has three forms of the double angle formula. Each is derived from the Pythagorean theorem, so you only need to know one (the rest can be derived from the formula and the Pythagorean theorem) . That said, one might suit the format of a particular problem than the others. For example, if you have a lot of sines in an equation, use the first form; Choose whichever one results in fewer calculations for you:
Example question: Factor and simplify the following expression:
cos4θ – sin4θ.
Step 1: Factor the expression:
The question asks us to factor, so let’s do that first to get it into a “double angle formula”- friendly format:
Step 2: Simplify:
- Use the basic trigonometric identity cos2 x + sin2 x = 1 to replace the left-hand side:
- Use the first cos double angle formula to replace the right-hand side of the formula:
The solution is cos 2θ.
 Double Angle Formulas and Half-Angle Formulas. Retrieved July 11, 2021 from: https://www.math.utah.edu/lectures/math1060/2-5PostNotes.pdf
 Joyce, D. (1996). Summary of trigonometric identities. Retrieved July 11, 2021 from: https://www2.clarku.edu/faculty/djoyce/trig/identities.html
Stephanie Glen. "Double Angle Formulas: Sin, Cos, Tan" From CalculusHowTo.com: Calculus for the rest of us! https://www.calculushowto.com/types-of-functions/trigonometric-function/double-angle-formulas-sin-cos-tan/
Need help with a homework or test question? With Chegg Study, you can get step-by-step solutions to your questions from an expert in the field. Your first 30 minutes with a Chegg tutor is free!