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How would I find the cosine of a fraction of a variable?

I am trying to find  the cosine of a fraction of a variable and I m lost. I can do one or the other but not combined. Any help would be greatly appreciated.

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Kay H. | Online Math and Science TutorOnline Math and Science Tutor
4.9 4.9 (565 lesson ratings) (565)
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!
Nathan C. | Math and Science (K through College)Math and Science (K through College)
5.0 5.0 (23 lesson ratings) (23)
More context is required to answer this question. What is the rest of the problem?