Could someone pls explain?
Part (ii) is just using some basic algebraic manipulation and identities you should know in order to express the equation entirely in terms of . Not sure what's confusing in their solution.
Then solving is the same as solving the quadratic you just derived since all the solutions will be exactly the same because all you do is just manipulate the eq. into a different form. It's the difference between solving and .
Once it comes to the at the end, all you need to be aware of is that cosine repeats itself every 360 degrees so we can use the same trick as we do with tan and just subtract/add 360 rather than 180 to get further solutions.
If it's the theory behind it you don't understand then TSR is not the best place to ask for help on that.