The document discusses the cosine law, which relates the lengths of sides of a triangle to the cosine of one of its angles. It defines the cosine law formula as c^2 = a^2 + b^2 - 2abcos(C), where c is the length of the side opposite angle C, a and b are the lengths of the other two sides, and C is the angle between sides a and b. Examples are provided to demonstrate how to use the cosine law to solve for a missing side length.