The sine rule, the cosine rule and area = ½ab sin C for triangles without a right angle, and how to choose between them.
What you will learn
- Choose the sine rule, the cosine rule or the area formula
- Find a side or an angle with the sine rule
- Find a side or an angle with the cosine rule
- Find the area of any triangle with ½ab sin C
Letters that face each other
Label the angles A, B, C and the sides opposite them a, b, c.
Side a faces angle A.
sine rule: a/sin A = b/sin B = c/sin C
cosine rule: a² = b² + c² − 2bc cos A
area = ½ab sin C
The rest of this lesson and its 8 practice questions are locked.
In this lesson
- Letters that face each other
- Which rule?
- The sine rule can give two answers
- Try it: Choose first
8 practice questions, marked as you go; about 25 minutes in all.
Key terms
Sine rule, Cosine rule, Between, Longer
In this unit: Pythagoras and trigonometry
Pythagoras' theorem, sin, cos and tan in right-angled triangles, angles of elevation, depression and bearings, exact values, 3D problems, and the sine and cosine rules.