Actually it is really clever. Look what they do on Wikipedia. They are clever enough to point that the distance between the circle's center and each corner on the polygon should be equal ( = the radius). In the next step they form a system of equations. You will have to solve 2 systems of equations: one for x and one for y. In each system you will have only one unknown. There are countless way to solve numerically system of equations.
There are determinants used on Wikipedia. They compute the main determinant, and later "partial determinants". The end result is division of "partial determinant"/"main determinant". It is not good way for numerical methods. You had better stick to other ways of solving this system of equations. I am sorry, but I don't know the native english term for "partial determinant" and "main determinant".
If you have only one type of polygon to crank out, you may be better off with derivation of your formulas, like you are trying to do now. However, if you want to do software, which is going to work with all cyclic polygons, you should do this approach with solving of system of equations.
Good luck!