File:Unit-circle sin cos tan cot exsec excsc versin vercos coversin covercos.svg - Wikimedia Commons
File:Unit-circle sin cos tan cot exsec excsc versin vercos coversin covercos.svg - Wikimedia Commons,Using unit circle to prove Sin(-x) = Sin(x) and Cos(-x) = Cos(x) - part 2,calculus - Intuitive understanding of the derivatives of $\sin x$ and $\cos x$ - Mathematics Stack Exchange