Sorry I did'nt help you when I first saw your thread. And that must be the reason for you to send me a privet message. That is because I just came home after a tiresome days work and I did'nt have that feeling to answer threads.
Anyway now do you know your mistake,
gives all the solutions that could be taken for . When writing the general solution you must take such that it is in the first quadrant. That is why I used . If this seems a bit confusing please refer,
General Solutions of Trigonometric Functions, Maths First, Institute of Fundamental Sciences, Massey University
Hope this will help you.