Explain displacement

x(t) is known as displacement and it denotes the position of a body at time. If the displacement is positive then that body is to the right of the chosen origin and if negative, then it is to the left. 

 If a body is moving with average speed v then in time t it will cover a distance d=vt. But, in fact, the speed of a car alters from time to time and so one should limit the use of this formula to small time differences only. So, more accurately, one explains an average speed over the small time interval  t Δ  as: