Which of the following is a formula for calculating total resistance in a parallel circuit?

In a parallel circuit, the total resistance is calculated using the formula RT = 1/(1/R1 + 1/R2 + 1/R3). This reflects the nature of how resistances behave when arranged in parallel. When components are connected in parallel, the overall resistance decreases because the current can flow through multiple paths. This results in a lower total resistance than any of the individual resistances.

The formula accounts for the reciprocal relationship between total resistance and individual resistances in a parallel configuration. By taking the sum of the reciprocals of each resistance, and then calculating the reciprocal of that sum, you determine the total resistance effectively. This illustrates how the addition of more pathways for current reduces overall resistance.

In contrast, other formulas listed do not apply to parallel circuits. The formula indicating the direct sum of resistances (R1 + R2 + R3) is pertinent for series circuits, where resistances add up to yield a total resistance that is simply the sum of each component's resistance. Similarly, the first formula (RT = ET × IT), which appears to relate total resistance to voltage and current, does not pertain specifically to the calculation of resistance in either series or parallel circuits.