# How do I know which trig formula to use in the exam?

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When faced with a problem that involves triangles, there is a simple flowchart you can follow to determine which formula or identity you need to use! Firstly you must ask yourself whether the triangle has a right angle. If it has, you must then ask if it has any other angles (or you are asked to find another angle), if you are you can use the standard trig formula of SOHCAHTOA, if it hasn't, then you can use Pythagoras' theorem c2=a2+b2. However, if you haven't got a right angle, you must decide if your triangle has a side length you know the opposite angle to (some call this SSA), or if you have an angle in between two known sides (some call this SAS). If you know the opposite angle you use the Sine rule, if the angle is not opposite a known side you use the Cosine Rule.

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