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In a series circuit, the total resistance is calculated by simply adding the resistance values of each individual resistor together. This means that if you have multiple resistors connected in a series arrangement, the total resistance can be found with the formula R_total = R1 + R2 + R3.

This method reflects the principle that in a series circuit, the current flowing through each resistor is the same, but the voltage drop across each resistor can be different based on its resistance value. As a result, the total resistance is a cumulative sum of all resistances, effectively creating a single resistance value that represents the entire circuit's resistance to current flow.

The other formulas provided represent different configurations or misunderstandings:

- The first formula, which suggests multiplying resistances, is relevant to parallel circuits, where total resistance becomes less than any individual resistance.

- The third formula is indeed for total resistance in parallel circuits, where the reciprocal of the total resistance is equal to the sum of the reciprocals of each individual resistance.

- The last formula, which involves subtracting resistances, does not apply to basic resistance calculations in either series or parallel configurations.

Understanding that total resistance in a series circuit is a simple addition helps solidify foundational concepts for analyzing electrical circuits.