You can use the Law of Cosines to find the length of the third side (a) of a triangle when you know the measure of an angle (A) and the lengths of the two sides (b and c) adjacent to that angle.First of all, I don't understand the law of cosines. I know the formula is a^2 = c^2 + b^2 -2cb (CosA), but I don't know how to use or apply it.