unit circle - SERP Analysis

https://en.wikipedia.org/wiki/Unit_circle

https://www.mathsisfun.com/geometry/unit-circle.html

https://www.uww.edu/documents/rock/academics/unit_circle_for_trigonometry.pdf

https://www.youtube.com/watch%3Fv%3DsUnjPeE8TWs

https://vmlc.tamu.edu/public-resources/trigonometry-series/trigonometric-functions-with-the-unit-circle

https://flexbooks.ck12.org/cbook/ck-12-precalculus-concepts-2.0/section/5.1/primary/lesson/the-unit-circle-pcalc/