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H3. The cosine rule. The cosine rule is a rule about the sides and angles in any triangle, not just a right-angled triangle. a2=b2+c2-2bccosA The rule tells you how to find the length of one side if you know the other two and the angle between them. An example Find the length of the side marked x. Use the same labelling of angles and sides as in the top diagram and substitute values into the cosine rule. x2=42+52-2.4.5.cos105 Evaluate x2, and hence x. x2=51.352... x=7.2