The Cosine Rule

Question 1

How do we know when to use the cosine rule?

Answer 1

We know to use the cosine rule if the triangle is not right angled and there is not a matching pair.

Question 2

What is the formula for the cosine rule?

Answer 2

In any triangle,

ABC a squared = b squared + c squared - 2bccosA

Question 3

How do we find the missing side of a triangle using the cosine rule?

Answer 3

To find the missing side of a triangle using the cosine rule we need 2 sides and an included angle between them.

Question 4

What is our thought process when we decide whether or not to use the cosine rule?

Answer 4

RAT? No
Matching pair? No
Use cosine rule.

Question 5

What do we need to find the missing angle theta? Where is the missing angle located in terms of a, b and c?

Answer 5

To find a missing angle we need to know all 3 sides.

The angle we need is included between sides b and c. Side a is always opposite the angle we are trying to find.

Question 6

What is the formula for finding a missing angle using the cosine rule?

Answer 6

The formula for finding a missing angle using the cosine rule is:
CosA = b squared + c squared - a squared
———————————–
2bc
Note this formula is not in this form on the formula sheet.