# The cosine rule

**The cosine rule**

For the following triangle, the cosine rule is as follows:

*a²* = *b²* + *c²* – 2*bc* cos A (this formula can be used to find side lengths)

this can also be written as:

cos A = *b²* + *c²* – *a²* / 2*bc *(this formula can be used to find angles)