solving a trigonometric equation for a variable

Hello there!

Well look, I'm a total axiom newbie and don't have much clue about math either. I really tried to solve this on my own, spent half a day to find what I need in the axiom documentation, but it's huge and I barely understand anything of it. Can you please help me out?

I need to deal with the following equation:

0.5 * v*v * cos(theta + phi) * cos(theta + phi) + g*l * cos(phi) = g*l

The variables v, theta, g and l are known. I need to calculate a phi that satisfies the equation above. So I would like to solve the equation for phi so that it starts with phi = ... This way any time I get a new set of v, theta, g and l, I can calculate a new phi. How can accomplish this using axiom? I tried:

solve(0.5*v*v*cos(theta+phi)*cos(theta+phi)+g*l*cos(phi)=g*l, phi)

The answer I get is:

 Cannot find a definition or applicable library operation named solve
      with argument type(s) 
                          Equation Expression Float
                                Variable phi
      
      Perhaps you should use "@" to indicate the required return type, 
      or "$" to specify which version of the function you need.

which I don't really understand either.

Can anyone please correct it? I just need one working example, so that I can use it for similar functions in the future.


Thank you,
Cappy