okay we get started by listing what may be of help here:

;

;

;

with the trigonometric identities:

First we assume

;

therefore we have

,

together with the application of the identities introduced above, which results:

(note that y is less than )

The identities also yield that

Therefore we may find the only real value of y (which isn't easy)

Hence

The bad news is that, I haven't yet figured out how to solve the equation without a calulator. So good luck on that, and I'll put my findings here as soon as I'm getting something.