The Cosine Rule
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Spotting the opposite side and the included angle

Spotting the opposite side and the included angle

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.

The Cosine Rule Lesson

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