The law of cosines is a formula related to SAS congruence.
It shows how to compute a side c once one knows the sides a and b and the included
angle C.
c2= a2 + b2 - 2ab cos C
This law is a generalization of the Pythagorean Theorem. When angle C is a right angle and thus cos C = 0, the formulas is the same as the Pythagorean Thereom.
The Law of Cosines is what permits the computation of the cosine of an angle by dot product of vectors.