problem with the polar commend

[insafba]

Hello there, 
I have a pretty long equation that I want to plot using polar coordinates.
Please see the equation below
  <http://octave.1599824.n4.nabble.com/file/t373159/equation_octave.png> 
The problem is when I write it in octave and trying to plot it using
polar(theta, r), the figure is empty!! nothing is plotted! 
could you please help me with this? thank you so much

here is the code 

theta= 0:0.02:2*pi;

r = sqrt(((b^2).*(cos(theta)).^2 + (a^2).*(sin(theta)).^2 -
abs((b^2).*(cos(theta)).^2 + (a^2).*(sin(theta)).^2)) ./
(2*(cos(theta)).^2.*(sin(theta)).^2));

polar(theta, r)



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