ABSTRACT
A.3 Law of Cosines The law of cosines, sometimes called the cosine law, is one of the most useful sets of sideangle-side relationships ever devised. You use the law of cosines to solve triangles that are not right-angled. You use it when you know any two sides and their included angle and you want to solve for the side opposite the angle. A variation of this law allows you to find the angles if you know only the sides. The law of cosines is also historically important. It is the fundamental equation for marine and aviation celestial navigation. It is used to determine directions and distances from port to port or to convert sights taken with a sextant to find a latitude and longitude position. Using the notation presented in Figure A.2, the cosine law for the sides of any spherical triangles (central angles) is expressed as
cos cos cos sin sin cos , cos cos cos sin sin cos , cos cos cos sin sin cos .
