# The Law of Cosines

The law of cosines $$c^2=a^2+b^2-2ab\cdot\cos C$$ is a generalization of the famed Pythagorean theorem to triangles that are not necessarily right. This formula is exceedingly useful in geometry, including in high school math and on math contests and olympiads. In our video, we prove the cosine law using geometric transformations.