The solution to the question is:
Why is the thevenin's equivalent Zth = R and not R-j as they are both in series?
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Sign up to join this communityWhy is the thevenin's equivalent Zth = R and not R-j as they are both in series?
They are not in series when you look at the circuit from Thevenin perspective.
Replace the sources with their equivalents (voltage sources are shorted because they have zero series resistance, current source are left open because they have infinite parallel resistance) and find the impedance seen.
As you can see from the solution, the 3V source is replaced with a short circuit so it shorted the capacitor, yielding the net impedance to be only R.
Why is the thevenin's equivalent Zth = R and not R-j as they are both in series?
The red-herring is the capacitor; it is totally in parallel with the right-hand voltage source and therefore, it plays no role in any analysis for Thevenin resistance.