Hello,

Note that

So

We know that an n-th root of the unity is in the form :

So you have to make appear that factor (2k) in order to find n.

Hence here, k=7 and n=12

Hmmm isn't it rather ?My answer is b).

Where is w ? Are you sure you copied it well ? because I find something like 40 :s

Not sure about this oneMy answer is c).