You can put this solution on YOUR website! Short answer: You can't. is an irrational number. Neither it nor it divided by a whole number like 8 can ever be expressed exactly (other than with and and .
But I think you left out part of the problem. I think you are supposed to find or or ...
which can be found exactly. (Hint: Use a "Half-Angle" formula.)
If my hint is not enough help for you to figure it out, please re-post the full problem.
P.S. If you are really sure that you posted the full, correct problem, then the problem is bad. To make sense the problem must ask you to find one or more of... or or ...
Are you sure it doesn't say to find all the trig function values for ?
Once you know which function(s) you are supposed to find, then you have the right idea with the Half-angle formulas.
You solution for has an error:
(Note: It is not necessary to change and/or into degrees to do these problems.)
(