Cosine rule

What is cosine rule?

The cosine rule relates the three sides and one angle of any triangle: a² = b² + c² − 2bc · cos A. Use it when you know two sides and the included angle (SAS), or all three sides and want to find an angle (SSS). For right-angled triangles, Pythagoras' theorem (cos 90° = 0) is the natural special case.