what is relative velocity in one dimension if va and vb are the velocities of two body a and b respectively then prove vab=va -vb

Relative velocity is nothing but the velocity of one body with respect to the velocity of some other body.  

Explanation:

Relative velocity may be defined as the rate at which the change of separation takes place between any two object.The relative velocity of any object A w.r.t. another object B when they are in motion is nothing but the rate of change of their position from one another.

Now, let the velocity of object A be  

         and velocity of object B is

Therefore, relative velocity of a object A when compared to the velocity of object B is given by .

hypothesising or postulating