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H3. The sine rule. The sine rule is a rule about the sides and angles in any triangle, not just a right-angled triangle. The rule tells you about the ratio of each side and the sine of the angle opposite to it. If you know some of the sides and angles, then you can use this rule to calculate the rest. 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 sine rule. Rearrange the equation to make x the subject.