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      Geometery and measure : Further trigonometry : The cosine rule

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      The cosine rule

      • Posted by sapayth
      • Categories Further trigonometry
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      s4-6-the-cosine-rule

      The cosine rule

      For the following triangle, the cosine rule is as follows:

      ys-s-4-6-1

      a² = b² + c² – 2bc cos A (this formula can be used to find side lengths)

      this can also be written as:

      cos A = b² + c² – a² / 2bc (this formula can be used to find angles)

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      Further trigonometry

      • Sin, cos and tan of any angle
      • Sin, cos and tan of 30°, 45° and 60°
      • Graphs of trigonometric functions
      • Area of a triangle using ½ab sin C
      • The sine rule
      • The cosine rule

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