The  Cosine Rule is used in triangles where one is trying to relate all  three sides to one angle. If you need to find the length of a side, you  need to know the other two sides and the opposite angle. 

Formula 

a2 = b2 + c2 – 2bc cos(A)

Side a is the one you are trying to find. Sides b and c are the other two sides, and angle A is the angle opposite side a

Example 

Christiano  Ronaldo is taking a shot at a standard 24 foot goal post. He is 32 feet  away from one goal post and 45 feet from the other. Through what angle  can a goal be made? 

Explanation : cosine rule

We will aply cosine tule to this problem because we know the three lengths.

24ft ^2  = 32ft ^2 + 45ft ^2 – 2(32ft)(45ft) cos θ θ = cos−1 (24 ft ^2 – 32 ft^2 – 45ft^2) / -2(32ft)(45ft) 

θ = 30.8312430

CR7 would have to shoot the ball around 31 degrees to score a goal. 

Sources 

Section  4: Sine and cosine rule. (n.d.). Retrieved August 11, 2022, from  https://www.cimt.org.uk/projects/mepres/step-up/sect4/index.htm