Where Re is the equivalent resistance of the parallel combination. R1, R2,... Rn are the resistors themselves. The most common error when calculating the equivalent resistance for parallel resistors is forgetting to inverse the summation.
Resistors, or any electrical component, in parallel all drop the same potential difference. In other words everything that make a parallel combination of resistors use up the same amount of voltage. This is because all the resistors in parallel are attached to the same two on both sides of the resistors. Since the wires on opposite sides of the resistors have different voltages, each resistor must use drop the same amount of voltage as the charge travels from on wire, through the resistors, to the other wire.
What is the equivalent resistance when a 4 Ω resistor is in parallel with a 12 Ω resistor.
by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)