Derive cosine angle sum & difference identities / addition & subtraction formulas

The sum & difference identities / addition & subtraction formulas are used to evaluate trigonometric ratios beyond the special angles (0, 30, 45, 60, 90 degrees), and are essential for small angle approximations. In this proof, the identity is derived on the unit circle, using Pythagoras' theorem and the cosine rule (law of cosines). Animation made with Python, Manim CE.

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