hey sinjan, first you have to know the concept of relative velocity.
usually, relative velocity is defined as the velocity of one body with respect to another.
now, the point is to understand that "what is the velocity of one body w.r.t. to other??"
well the ans. is simple. it is velocity of one body when the other is assumed to be at rest. this is what they've done in the highlighted part, they've made the velocity of body B equal to zero to find the relative velocity of A w.r.t. B...
Moderation Team
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