For any triangle ... a, b and c are sides. C is the angle opposite side c. the Law of Cosines (also called the Cosine Rule) says:

Here is everything you need to know about the law of cosines or cosine rule. You’ll learn how to use the cosine rule to find missing sides and angles in an oblique triangle (non-right triangle) …

We use the Law of Cosines to solve an oblique triangle or any triangle that is not a right triangle. When solving an oblique triangle, you are trying to find the lengths of the three sides and the …

Mar 10, 2025 · The cosine rule is a commonly used rule in trigonometry. It can be used to investigate the properties of non-right triangles and thus allows you to find missing information, …

Jul 23, 2025 · In this article, we will discuss the introduction, definition, properties, formula of the Cosine Rule, and its meaning. We will also understand the proof of the Cosine Rule.

Revise how to use the sine and cosine rules to find missing angles and sides of triangles as part of National 5 Maths.

Here’s when to use cosine rule: Got two sides and the angle between them? That’s the SAS case, and cosine rule is perfect for this. Know all three sides and need to find an angle? That’s the …

When to Use the Law of Cosines The Law of Cosines is particularly useful in two main scenarios when solving for unknown sides or angles in any triangle (including non-right triangles):

Since cos 90º = 0, we are left with the Pythagoras theorem. The law of cosine is also known as the cosine rule. This law is useful to find the missing information in any triangle. For example, …

The problems below are ones that ask you to apply the formula to solve straight forward questions. If they start to seem too easy, try our more challenging problems.