Hi Cleveland,
The attached notebook revises your code. I have corrected some syntax issues, and used rules rather than Set (=) to define constant parameter values.
You will see that FindRoot does indeed provide solutions for a for 2 of your b values. In the other 2 cases it returns convergence warnings and invalid solutions. At the very end, all 4 {a,b} pairs are checked, and it is seen that the first and third are correct solutions, but the second and fourth are not.
You will also see that the 2 valid solutions are both complex. If all coefficients are Real, FindRoot will only return real solutions. However, you will see in a part of the notebook that, for your parameter values, FindRoot is indeed being given an equation with complex coefficients. Unless you expect complex solutions, you might want to check that the equations and parameter values are correct. (My experience with LC pretilt is that it is not a complex number.)
Best regards,
David
*** Edit: Sorry -- I attached the wrong file. It has been updated.
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