6.2 Law of Cosines

You can solve an oblique triangle if you have all three sides (SSS) or two sides with their included angle (SAS) using the law of cosines.

Law of Cosines

Proof of Law of Cosines

http://pages.pacificcoast.net/~cazelais/173/law-sines-cosines.pdf

(page two)

The Law of Cosines can also be used to establish the following formula for the area of a triangle. This formula is called Heron's Area Formula.

Given any triangle with sides of lengths a, b, and c, the area of the triangle is: