how do I find the values of these expressions (arctan, arccos, etc)?

I'm not just trying to get answers, I really don't know how to do this :(
the 2 questions are:

arccos(cos(9/8 pi))

arctan(tan(11/9 pi))

cos(tan inverse (square root 3 /3))

thank you!
*3 questions [:
 
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