I've no idea what was in your ugly pink and black textbook, but I'd have thought those trigonometric formulae would include lots of things like cos(A+B) = cos(A

I've no idea what was in your ugly pink and black textbook, but I'd have thought those trigonometric formulae would include lots of things like cos(A+B) = cos(A)cos(B) - sin(A)sin(B). How do you "reduce that to a ratio using the SOHCAHTOA principle"? By drawing a diagram with two right-angled triangles stuck together and doing the relevant fiddly geometry? That seems like vastly more trouble than remembering the formula. (It took me a minute to work out how to derive the formula at all that way, and I'm a pro.)