Law of cosines - Wikipedia
https://en.wikipedia.org/wiki/Law_of_cosines
OverviewUse in solving trianglesHistoryProofsIsosceles caseAnalogue for tetrahedraVersion suited to small anglesIn spherical and hyperbolic geometryIn trigonometry, the law of cosines (also known as the cosine formula or cosine rule) relates the lengths of the sides of a triangle to the cosine of one of its angles. For a triangle with sides and opposite respective angles and (see Fig. 1), the law of cosines states: The law of cosines generalizes the Pythagorean theorem, which holds only for
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