Law of cosines
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Revision as of 10:35, 1 June 2011
The law of cosines is a formula that helps in triangulation when two or three side lengths of a triangle are known. The formula relates all three side lengths of a triangle to the cosine of a particular angle.
When to use it: SAS, SSS.
Let be oriented so that is at the origin, and is at the point.
is the distance from to .
Substituting the appropriate points into the distance formula gives us
Squaring the inner terms, we have
Square both sides for
Complete the triangle using the law of cosines.