GCSE Maths (AQA)

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The Cosine Rule

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Geometry and Measures Mensuration and Calculation

The Cosine Rule

3:38 Triangle Rules
Spec G1 G22
Previous The Sine Rule Next General Area of a Triangle
  • The cosine rule allows you to calculate angles and sides of non-right angled triangles
  • The formula for the cosine rule is A^2 = B^2 + C^2 - 2BCCosA
  • Lowercase letters represent sides and uppercase letters represent angles
  • The cosine rule can be used when you know all three sides or two sides and the angle between them
  • The cosine rule can also be rearranged to calculate an angle: CosA = (B^2 + C^2 - A^2) / 2BC
  • To find an angle using the cosine rule, you can use the inverse cosine function
  • An example problem is given to demonstrate the use of the cosine rule to find an angle in a scalene triangle
  • The angle x is calculated to be 64.6 degrees to three significant figures

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