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I'm having a senior moment. I know the 3db frequency of an RC LPF is 1/(2*pi*R*C). My memory is that you can solve for that by setting R = C. So:

R = magnitude(1/JwC)

R = 1/(wC)

R = 1/(2*pi*F*C)

F = 1/(2*pi*R*C)

So that all seems right. But if R = magnitude of C doesn't that mean your gain is 0.5? (voltage divider where both top and bottom elements are the same).  20*log(0.5) = 6db, not 3db!

I'm getting the right answer but it doesn't feel to me like it should be right. What am I missing? Thank you!