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For a triangle with sides a, b anc c, where A is the angle opposite side a, B is the angle opposite side b, etc.:

cos A = ( b2 + c2 - a2 ) / ( 2 bc )

cos B = ( a2 + c2 - b2 ) / ( 2 ac )

cos C = ( a2 + b2 - c2 ) / ( 2 ab )

These are just rearrangements of the ordinary cosine rule:

a2 = b2 + c2 - 2 bc cos A

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Q: How do you find unknown angles if you know 3 sides?
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