GCSE Maths (Edexcel)

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The Sine Rule

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Geometry and Measures Mensuration and Calculation

The Sine Rule

4:39 Triangle Rules
Spec G1 G22
Previous The Tangent Graph Next The Cosine Rule
  • The sine rule allows you to calculate angles and sides in a triangle that is not a right angle triangle.
  • The sine rule can be written as lowercase a over sine of capital A equals lowercase b over sine of capital B equals lowercase c over sine of capital C.
  • Lowercase letters represent sides and uppercase letters represent angles.
  • To use the sine rule, you must know the length of a side and the angle opposite, as well as one other angle or side.
  • The sine rule can be rewritten so that the sine of each angle is on the numerator and each side length is on the denominator.
  • The second form of the sine rule is easier to use if trying to work out an angle.
  • Example: Using the sine rule to calculate the length of side x in a scalene triangle.

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