Work Step by Step
It comes from the pythagorean theorem applied to a circle of radius 1. The catheti are $sin~x$ and $cos~x$, and the hypotenuse is $1$ (the radius).
You can help us out by revising, improving and updating this answer.Update this answer
After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.