The square of a side of a triangle is equal to the sum of the squares of the other two sides, minus double the product of the latter two sides and the cosine of the angle between them:
Here are the sides of the triangle and is the angle between and .
Cosine theorem. Yu.A. Gor'kov (originator), Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Cosine_theorem&oldid=13758