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# Law of Cosines

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Just a quick question- when is the Law of Cosines $$c^2=a^2+b^2+2ab\cos(C)?$$ I thought it was if the triangle you were using the Law of Cosines on was obtuse, but I'm confused because when I browse on the internet it says it always is $$c^2=a^2+b^2-2ab\cos(C)$$

Jan 7, 2021

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It is always with -2ab*cos(C). When C is obtuse, then cos(C) is negative and so -2ab*cos(C) is positive.

You are very welcome!

:P

Jan 7, 2021