Square both sides:
Now , so
Now to clean some stuff up.
This is a quadratic in . If you have a hard time seeing that (or dealing with it) let :
So I get
Now, if this isn't enough to have to clean up, you also have to make sure that you only take a solution that has a magnitude of one or less (because .) If you have numbers, now would be a good time to plug them in.
I did a bit of work on this and got:
I'd advise checking it to make sure I didn't make a mistake.
(I feel this equation is relatively familiar. Where did it come from?)