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 $a,b,c$ are the sides of the triangle and $C$ is the angle between $a$ and $b$.
Cosine theorem. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Cosine_theorem&oldid=31396