# The Cosine Law by Similarity

The *Law of Cosines* establishes a relationship between the angles and the side lengths of \(\Delta ABC\):

\(c^{2} = a^{2} + b^{2} - 2ab\cdot \mbox{cos}\gamma,\)

where \(\gamma\) is the angle in \(\Delta ABC\) opposite side \(c\).

This proof without words submitted by Anders Kaseorg combines together \(3\) similar triangles; but where is the third? The proof was actually published at the *American Mathematical Monthly*, 121, February 2014, p 149, by Miles Dillon Edwards.

(There are several theorems that are proved by similar technique.)

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