The law of cosines can be used in two circumstances. First, we can use it to find the missing side of a non-right triangle when we are given a side, an angle in the middle, and another side. Or, we can use it to find a missing angle of a triangle when we are given all three sides of a non-right triangle.
The law of cosines is:
c^2 = a^2 + b^2 - 2abcos(c) where a and b are the two shorter sides of the triangle, c is the longest side, and cos(c) is either the given angle or the missing angle (if you have all three sides).
It does not matter what side you label a or b, just make sure that c is your longest side!
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