You need to use trig identities. There is the double angle cos 2€ and 1/2 angle identity cos 1/2€. If you can get your fraction to fit into one of these formulas you should be able to solve it from there. For example, if you have cos 1/4@ you could rewrite it as cos 2(1/8)@. The identity tells you that cos 2@ = cos^2@-sin^2@. So it would be cos^2(1/8) - sin^2(1/8). I hope this helps you figure out the answer whatever your fraction is!