Cosine law : The cosine law states that in any triangle we have $c^2=a^2+b^2-2ab\cos \gamma.$ In can be used to calculate the length of the third side if the lengths of two sides and the angle in between those two sides is given. It can also be used to calculate angles if all three side lengths are given. The Pythagorean theorem is the special case of the cosine law with $\gamma=90^{\circ}.$