My book shows three rules for the Law of Cosines:
a2 = b2 + c2 - 2bc cos A
b2 = a2 + c2 - 2ac cos B
c2 = a2 + b2 - 2ab cos C
Don't these all really mean the same thing? "To find (the square of) the length opposite an angle, square the other two sides, and subtract two times the two lengths times the cosine of that angle."