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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

Step-by-step lessons on CCEA GCSE Maths The Cosine Rule. Each one builds up to exam-style questions. Build fluency with number, ratio and algebra first, since geometry, probability and statistics topics rely on them, and leave extra time nearer the exam for Higher-tier problem-solving and multi-step reasoning questions.