Although this is a physics problem, the problem is in regard to vectors.
There is the relevant reference to this image here:
What I need to find is the unit vector of force $F_1$. The book gives it as the following: $$u_{F_1} = sin 35° cos 20° i - sin 35° sin 20° j + cos 35° k $$ I would like to know where this set of cosines/sines comes from, and how do I get it? Where does the $sin 35° cos 20° i$ come from? Shouldn't it be (in my understanding of the picture) $cos 20° i$? (Same question for the j and k unit's as well) On top of the that, is this a general form equation? How do I know which angles to pick?