When do I use the sin rule and when do I use the cosine rule?

The sin rule is used when you need to find a length of a side or an angle. You use the sin rule when you're given an angle, an opposite side length and also an angle or side length opposite to the one you're trying to find. The cosine rule is used when you're given a triangle with two sides and an incuded angle OR when you're given three side lengths. The cosine rule also can be used to find a side length or angle but you have to be careful to label your triangle and use the formula correctly. When you have three side lengths it's easier to use a rearranged version of the cosine formula.