When using trigonometry to calculate side lengths/angles, how do you know which identity to use?

Choosing whether to use sin, cos or tan is what will gain you the majority of marks for a trig question. Firstly, you should label the three sides of the triangle as hypothenuse, adjacent and opposite. In a right angled triangle, the side opposite the right angle (the longest side) is the hypothenuse. The other two sides are opposite and adjacent, depending which angle you're working with. SOH CAH TOA is an easy method for identifying which to use: Sin(angle)-Opp/Hyp Cos(angle)-Adj/Hyp and Tan(angle)-Opp/Adj. Then simply substitute in the two values given in the question to calculate the unknown. Can be used to calculate a side or an angle.