Seriously, when do you use either/
Remember the acronym Sohcahtoa.... it tells you what you need to do, know or compare to.
Sin=opposite side over hypotenuse (Soh)
Cos = adjecent side over hypotenuse (cah)
Tan = Opposite side over Adjacent (toa)
The are relationships of the lengths of the sides to the angle that forms them. If you know any two of the 3 parts of one of those equations, you can solve for the remaining piece.
