Question

according to 12.31 delta r =rB-rA but when we use it we use delta r=rA - rB why??
please help me to clear my confusion and i had underlined problem​

Answer 1

Answer:

because in first case charge is moving from point A to B so , ∆r = rB-rA

in second case charge is moving from B to A so, it is ∆r = rA -rB