For an RC circuit, impedance Z is given by:
Z = 1/wC, where w is omega and C is capacitance
Now w = 2πf, where f is frequency
So Z = 1/2πfC
This means that impedance Z is inversely proportional to frequency f.
So Z1/Z2 = f2/f1
Since f1 = f and f2 = 2f:
f2/f1 = 2f/f = 2
So Z1/Z2 = f2/f1 = 2
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