Derive spherical mirror formula using Fermat's principle

[Bhumble]

Homework Statement

Using Fermat’s principle, derive the spherical mirror formula in paraxial approximation:
[tex]\frac{1}{s_o} + \frac{1}{s_i} = \frac{-2}{R}[/tex]
where so and si are object and image distances, R is the radius of curvature of the sphere.

Homework Equations

As far as I know you are just suppose to use geometry and possibly some trig.

The Attempt at a Solution

I drew a sketch with a concave mirror. I have the theta = 0 angle traveling a distance A. I'm unsure what to equate the other path that reflects off of a curved portion of the mirror.

 

[Bhumble]

Nevermind. I found an example in the book and I'm pretty sure I have it now.