How do I know when to use the cosine or sine rule?

Both the cosine and sine rule are used to find either a missing side length or angle of a triangle that doesn't have a right angle. The cosine rule is used when you know the length of two sides and the angle between them. You can then use this rule in its original form to work out the remaining length, or the formula can be rearranged and used to find an angle if you know the lengths of all three sides.The sine rule can be used to find the length of a side when two angles and an opposite side lengths are known, using the original form of the sine rule. It can also be used to find an angle is two lengths of sides and an opposite angle are known, but formula must be rearranged to find this.