Complex sentence: Both reactance\\ scriptstyle and resistance\\ scriptstyle are required to calculate the impedance\\ scriptstyle, although in some circuits one of these may dominate: an approximate knowledge of the minor component is useful to determine if it may be neglected.
Split into simpler sentences: Both reactance\\ scriptstyle and resistance\\ scriptstyle are required to calculate the impedance\\ scriptstyle. In some circuits one of these may dominate, but an approximate knowledge of the minor component is useful to determine if it may be neglected.