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To use the Sine Rule

You need one complete ratio (Side to its opposite angle)

a/sinA = b/sinB = c/sinC OR sinA/a = sinB/b = sinC/c

This is used to find either a side or an angle in a non right-angled triangle. This is the Sine Rule. You do not need to use all three ratios, just one that you know and the one that you need to complete.

To use the Cosine Rule

You need to have two sides and the included angle

a²=b²+c²-2bc(cos)A

This is the equation for the Cosine Rule, it can be changed to have b or c as the subject (e.g. b²=a²+c²-2ac(cos)B).

You should use the Sine and/or Cosine Rule

When the triangle is non right-angled

You should use SOHCAHTOA

When the triangle is right-angled