(a) State the expression for the straight-line distance between two points P(x1,y1,z1) and Q(x2,y2,z2) located in three–dimensional space.
(b) Give a step-by-step explanation of how this spatial distance formula reduces to the familiar two–dimensional expression when both points happen to lie in the same plane parallel to the xy-plane. Your explanation should clearly describe the geometric reasoning involved and identify the condition that links the two situations.