## Precalculus (6th Edition) Blitzer

The Law of Cosines gives $c^2=a^2+b^2-2ab\ cosC$ In the case of a right triangle $C=90^\circ$, we have $cosC=0$ and the Law of Cosines becomes $c^2=a^2+b^2$, which is the Pythagorean Theorem. Thus, the Pythagorean Theorem is a special case of the Law of Cosines.