First I am assuming f is a constant, so I'll replace 1 + f with k for simplicity.
Get rid of the fraction. ie
Now expand the
Now isolate the term and replace it with . Square both sides and collect like factors. You will now have a quadratic by which you can solve by the quadratic formula. Then take the inverse cosine of both sides.
I may/may not have some errors in there. But the basic method is still the same.