1. The problem statement, all variables and given/known data I'm having a problem with this. I was told to use the difference of sums formula. I'm not exactly sure of what that is/means. 2. Relevant equations Difference of sums? 3. The attempt at a solution I'm thinking that Cos(a)=1/3 refers to 1 being the adjacent angle and 3 is the hypotenuse. Therefore the opposite angle equals to sqrt(32-12). Or does 1/3 somehow correspond to an angle? a or alpha should be 30 degrees then right? I need a lead-in. From there, I think that I'll be able to take it form there. Than you in advance.