# Cosine Rules

Related Topics:
More Lessons for A Level Maths
Math Worksheets

Examples, videos, solutions, activities, and worksheets that are suitable for A Level Maths.

What is the Sine Rule?

For sides, $$\frac{a}{{\sin A}} = \frac{b}{{\sin B}} = \frac{c}{{\sin C}}$$

For angles, $$\frac{{\sin A}}{a} = \frac{{\sin B}}{b} = \frac{{\sin C}}{c}$$

What is the Cosine Rule?

To find a side when you know the other two sides and the angle between them use:
$${a^2} = {b^2} + {c^2} - 2bc\cos A$$

To find an angle when you know three sides use:
$$\cos A = \frac{{{b^2} + {c^2} - {a^2}}}{{2bc}}$$

Sine and Cosine Rule
Sine and Cosine Rule - Exam Questions

Cosine rule (Finding a Length)
How to find the length of a side of a triangle using the cosine rule?
Cosine rule (Finding an Angle)
How to find an angle of a triangle using the cosine rule?
A-Level Maths Edexcel C2 January 2008 Q6a ExamSolutions
This question is on the cosine rule A-Level Maths Edexcel C2 January 2007 Q9(a) : ExamSolutions
Worked solution to the above Core 2 question on finding an angle in radians using the cosine rule.

Try the free Mathway calculator and problem solver below to practice various math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.