What is the cosine rule?

What is it?The cosine rule is a tool to find the size of an angle or the length of a side in a triangle.When to use it?- When we are given 2 sides and an angle- When we are given 3 sidesHow?For a triangle with sides a,b and c and angles A, B and C the cosine rule can be written as:a² = b² + c² - 2bc cos Aorb² = a² + c² - 2ac cos Borc² = a² + b² - 2ab cos C