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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 (577 lesson ratings) (577)
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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)
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More context is required to answer this question. What is the rest of the problem?