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law of cosines

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use the law of cosines.  its pretty straight forward.  Usually when you learn the law of cosines you learn that

c^2 = a^2 + b^2 - 2ab cos C.  You use this form if you have angle C.  If you have angle A, which is what your question is asking, you use:

a^2 = b^2 + c^2 - 2bc cos A

and if you are given angle B then you use:

b^2 = a^2 +c^2 - 2ac cos B

 

Good luck