We are given that co-ordinates of P is (x1,y1) and Q is (x1,y1).Distance between the points P (x1,y1) and Q (x1,y1) is
PQ = √(x2−x1)+(y2−y1)2 ... (i)
(i) When PQ is parallel to y-axis then x1 = x2 from (1), we have
PQ = √(x2−x1)2+(y2−y1)2
= √(x1−x2)2+(y2−y1)2 = |y2−y1|
(ii) When PQ is parallel to x-axis, then y1 = y2 from (1), we have
PQ = √(x2−x1)2+(y2−y1)2
= √(x2−x1)2+0 = |x2−x1|