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Say we got this circuit:

Initially we had the X resistance, but then I opened the circuit in the terminals of resistance X, and I moved the resistance X to the equivalent circuit (my teacher says this is a shortcut of how doing it). My doubt is about calculating the equivalent resistance: it is obvious that R1 is in series with R3, and then they are in parallel with R2. But, since I opened the circuit, the current that are going to those terminals is going to be zero, so there are no nodes, because the current doesn't divide. So, why couldn't I say that R1 and R2 and R3 are in series, or R1 and R2 are in series and these are in parallel with R3.

What's missing me? Doesn't the current in terminals is zero?

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First it would be really helpful to see the question you are trying to solve so I can give you a better explanation, but I think I see your confusion. The terminals are not actually an open circuit as you are assuming, in an example like this you can imagine that the two terminals connected to some unknown circuit and that they are providing a potential across the circuit you have drawn. Basically the undrawn portion of the circuit is the equivalent of a battery.

You asked about R1, R2, and R3 being in series, this is correct if and only if your terminals really are not connected to any voltage source. But this would result in a loop with no inputs or outputs, in short a very useless circuit not worth examining. S I doubt this is where your question is leading

What is most likely is you are looking at a voltage or current divider question. If that is the case please provide some more context to your circuit and I will show you how to solve it.

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