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The Cosine Rule

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Lesson

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9 minute activity

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Triangle ABC with angle A highlighted, side a opposite angle A, and sides b and c meeting at the included angle

The Cosine Rule is used in a non-right-angled triangle when you know two sides and the included angle, or when you know all three sides. It is the right choice when Pythagoras and SOHCAHTOA do not fit.

In triangle ABCABCABC, side aaa is opposite angle AAA, side bbb is opposite angle BBB, and side ccc is opposite angle CCC. The included angle is the angle between the two known sides.

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41 exam-style questions

Practice questions

Question 1

3 marks

A triangle is shown with two sides of lengths 13 cm and 10 cm. The angle between these two sides is 100∘100^\circ100∘. The third side of the triangle is labeled x x\,x cm.

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22 flashcards

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Which angle must be known to find a missing side using the Cosine Rule?

The Cosine Rule Revision Guide

  1. GCSE
  2. /Maths
  3. /The Cosine Rule

Revision notes for Edexcel GCSE Maths The Cosine Rule. Open the guide for explanations and worked examples. Written against the Edexcel GCSE Maths (1MA1) specification, so the content matches what's examinable rather than general Maths background.

Revision guides

Practise questions

1 of 5

Two sides of a triangle are 6 cm and 8 cm, and the angle between them is 50∘50^\circ50∘. Which side can be found directly using the Cosine Rule?