Editing Trigonometric functions (section)

====Law of cosines====
{{Main|Law of cosines}}
The law of cosines (also known as the cosine formula or cosine rule) is an extension of the [[Pythagorean theorem]]:
<math display="block">c^2=a^2+b^2-2ab\cos C,</math>
or equivalently,
<math display="block">\cos C=\frac{a^2+b^2-c^2}{2ab}.</math>

In this formula the angle at {{mvar|C}} is opposite to the side&nbsp;{{mvar|c}}. This theorem can be proved by dividing the triangle into two right ones and using the [[Pythagorean theorem]].

The law of cosines can be used to determine a side of a triangle if two sides and the angle between them are known. It can also be used to find the cosines of an angle (and consequently the angles themselves) if the lengths of all the sides are known.