law of cosines

Question

use the law of cosines to find the side of each triangle. round the answer to nearest tenth.
angle(A ) = 58 degree
 side (b) = 9
side (c) =12
find side (a) ?

Answer 1

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