Sine Or Cosine Rule? | Trigonometry | Maths | FuseSchool
Channel: FuseSchool - Global Education
Date: 2017-03-08

Not every triangle is a right-angle triangle, so we can't always use Pythagoras and SOHCAHTOA to find missing sides and missing angles. We instead use the sine rule or the cosine rule. They can both be used to find either missing sides or missing angles in any triangle (right angle or not). We label the sides as a, b and c, and angles as A, B and C. 

We need to work out whether to use the sine rule or the cosine rule. 

The sine rule is easier, so look for that one first. The sine rule is based on opposite pairs: you need an angle and the side opposite it, then you need either an angle and you want the opposite side OR you need a side and you want the opposite angle. 

If we don't have the right combination of sides and angles for the sine rule, then we can use the cosine rule. For the cosine rule, we either want all three sides and to be looking for an angle, OR we need to have a side, angle and side all next to each other, and to then be wanting to find the third side.

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