## Law of cosines

I saw on a website, law of sines are transposed. Example:

c^2=a^2+b^2-2abcos(C)

Transposed: cos(C)= a^2+b^2-c^2/2ab

I need a proof how it became this way.

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I saw on a website, law of sines are transposed. Example:

c^2=a^2+b^2-2abcos(C)

Transposed: cos(C)= a^2+b^2-c^2/2ab

I need a proof how it became this way.

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What is the perimeter of the value of the curve i=3(1 +cos x)?

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