Course: Trigonometry
Topic: Oblique Triangles and the Laws
Subtopic: Law of Cosines


Continuing our study of oblique (non-right) triangles, in today's lesson we study the second of the two laws used to solve triangles, the law of cosines. It is used when two sides and the included angle be given, enabling us to find the third side, or three sides be given, enabling us to find an angle. In either case the law of sines is then used to complete the triangle.


By the end of this topic you should know and be prepared to be tested on:


Terms you should be able to define: included angle, law of cosines

Memorize the Law of Cosines!
All three forms are shown below using standard triangle notation.

Mini-Lectures and Examples

STUDY: Oblique Triangles - Law of Cosines & Area of a Triangle

rev. 2020-10-31