You can do it in a long way (I replace by ).
First we're going to expand and in terms of , therefore:
Therefore the equation becomes:
I've found that in the first part of the question
Now I have to solve for theta
I can see straight away that 0 and multiples of pi are my solutions,
But I don't know how to do this problem in a methodical way.
I thought there would be substitution involved but I can't see how.