So, firstly, make sure you label your triangle correctly! Angle A is opposite side a and so on:

When can you use Sine rule?

If you look at the fractions for Sine rule, you will only need 2 of the fractions the a/sin A and b/sin B.

There are 4 unknowns in those 2 fractions.

2 angles and 2 side lengths that are opposite.

You can only solve an equation where one thing is unknown.

This means if you have a question that requires sine rule, the question has to give you the following information:

2 side lengths and an angle that is opposite one of those sides

Or

2 angles and a side length that is opposite one of those angles.

It won’t work if you have 2 side lengths and the angle in between them – that’s cosine rule.

It won’t work if you only have 3 side lengths – again that is cosine rule.

More questions and explanations coming soon!