**Law of cosines - Wikipedia**
https://en.wikipedia.org/wiki/Law_of_cosines

In trigonometry, the law of cosines (also known as the cosine formula, cosine rule, or al-Kashi's theorem ) relates the lengths of the sides of a triangle to the cosine of one of its angles. Using notation as in Fig. 1, the law of cosines states where γ denotes the angle contained between sides of lengths a and b and opposite the side of length c. For the same figure, the other two relations are analogous:

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