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Law of cosines

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For an arbitrary ΔABC the law of cosines states:

a2 = b2 + c2 - 2 ∙ b ∙ c ∙ cosA

b2 = a2 + c2 - 2 ∙ c ∙ a ∙ cosB

c2 = a2 + b2 - 2 ∙ a ∙ b ∙ cosC

or generally as:

cosC = (a2 + c2 - c2) / 2ab

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