10.2.3—Kirchhoff’s laws, a formula for the combined resistance of two or more

The combined resistance of a network is the single resistance that gives the same total current for the same applied voltage.

Reduce simple series or parallel sections step by step, preserving which elements share current or p.d. before applying the next rule.

Two 6 Ω resistors in parallel have equivalent resistance 3 Ω, then adding a 2 Ω resistor in series gives 5 Ω total.

Equivalent resistance is not the arithmetic sum for every network; parallel branches always give a value below the smallest branch resistance.