Need Help Solving This Really Difficult Quadratic Equation!

Hi all this relates to a Digital Signal Processing question, but it's the maths I can't understand when attempting to solve the quadratic equation.

This was originally part of a transfer function (denominator) and basically reads:

$\displaystyle 1-2rCos(\omega_cT)z^-1 + r^-2$

Multiplying this line to positive values of z, to give the quadratic the NOW equation. I'm attempting to solve for r:

$\displaystyle z^2-2rCos(\omega_cT)z^1 +r^2$

a=1

b=$\displaystyle 2rcos(\omega_cT)$

c=$\displaystyle r^2$

But this where I fall on, and where I need help, because I cannot for the life of me solve with another unknown in it.

$\displaystyle (2rcos(\omega_cT) +/- [sqrt](-(2rcos(\omega_cT))^2-4r^2)$

all divided by 2

I can change the $\displaystyle b^2$ in the brackets to become = $\displaystyle 4r^2cos^2(\omega_cT)$ but then I've become snookered. Can anyone help? I can't go any further unfortunatley.

Apologies my Mathtags are terrible! (Can't do the quadratic properly).