# Week 18 – precalculus 11

This week in precalculus 11 I learned about cosine law. Cosine Law is to solve triangles that are not right triangles. We should know that we use the sine law when we have given two angles and one side, or two sides and a non-included angle, and we use the cosine law when we are given three sides, or two sides and the included angle.

The formula for cosine law is:

${a^2}$ = ${b^2}$ + ${c^2}$ – 2bc cosA

Here is an example: