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The Cosine Rule

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.

Questions

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36 exam-style questions

Practice questions

Question 1

3 marks

A surveyor measures the distances between three markers, AAA, BBB, and CCC, placed on a plot of land. The distance AB AB\,AB is 6.4 m, the distance BC BC\,BC is 5.5 m, and the distance AC AC\,AC is 8.1 m.

Flashcards

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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 CCEA GCSE Maths The Cosine Rule: explanations and worked examples.

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?