How do I know when to use sin, cos or tan?
- Check that your triangle is a right-angled triangle. Remember, you can only use pythagoras theorem or SOHCAHTOA on right-angled triangles.
- Label the sides O, H and A: opposite (the angle), hypotenuse (longest side), adjacent (to the angle). Label the opposite and hypotenuse first, that way you won't make a mistake on the adjacent which some students find confusing.
- Cross out the irrelevant side and write out SOHCAHTOA.
- Which combination do you have: OH, AH or OA? OH: use sine. AH: use cosine. OA: use tangent.
- Write out the formula relevant to the question: sin xo = O/H or cos xo = A/H or tan xo = O/A
- Substitute the values given in the question. There should only be one unknown.
- Solve for the unknown by rearranging the equation.
