ok i'm having trouble with this problem...

i think there must be a very easy way of doing it but i'm just not getting it.....( i ended up in a huge mess)

tan(4x)=1.....sounded really easy to me!

now,

which is

and i used the same expansion method to expand cos(4x) and tried to solve but i still don't see the answer which has to be solved algebrically

help is appreaciated...

thnx