# Thread: when would you use cosine and sine law

1. ## when would you use cosine and sine law

i'm unsure of in what situations i am supposed to use the sine or cosine law. can someone please explain to me when to use which?

2. Originally Posted by euclid2
i'm unsure of in what situations i am supposed to use the sine or cosine law. can someone please explain to me when to use which?
Look at the formula for each rule. What angles and sides appear in them? That should tell you when each should be used.

3. Sine law is if you know one side and its opposite angle. Once you know this ratio, you can work out others if you have another angle or side.

Cosine law is if you know two sides and the angle between them.

Even though the equation is slightly less simple, I prefer using the cosine law when solving triangles since inverse cosine returns an angle between 0 and 180. If you use the sine law, you will have to use the inverse sine and that could cause problems since inverse sine returns a value in the range -90 to 90, e.g. if you're trying to find an angle = 95 degrees. Inverse sine will give you 85 degrees instead of 95 degrees, which could be quite difficult to spot.