How do I know whether to use sin, cos, or tan in trigonometry?

Label the sides of your triangleHypotenuse is the longest (opposite right angle)Opposite is opposite the given angleAdjacent is next to the given angleTwo are useful, one is irrelevant. Cross out the one that we do not know, or are not looking for.Two sides will remain labelled. Write out the SOH CAH TOA triangles and see which contains these two sides.This is the triangle you will use for the calculation.