Relationships between angles on the unit circle
The formulae for adding and subtracting angles are derived from the relationships between angles on the unit circle.
Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"Algebra and Geometry."
http://cnx.org/contents/13ac107a-f15f-49d2-97e8-60ab2e3b519c@5.241
OpenStax
CC BY 4.0.