What Is the Cosine Rule?
The cosine rule is a formula commonly used in trigonometry to determine certain aspects of a non-right triangle when other key parts of that triangle are known or can otherwise be determined. It is an effective extension of the Pythagorean theorem, which typically only works with right triangles and states that the square of the hypotenuse of the triangle is equal to the squares of the other two sides when added together (c2=a2+b2). The cosine rule is an extension of this mathematic principal that makes it effective for non-right triangles and states that in regard to a certain angle, the square of the side of the triangle opposite that angle is equal to the squares of the other two sides added together, minus two times both of those sides multiplied together with the cosine of that angle (c2=a2+b2-2ab cosC where C is the angle opposite side c).
