Please explain how one would determine if a system is undergoing constant acceleration based on knowing the average velocity.

Suppose an object starts at time t=0 at a position defined as x=0 m. At t =.5 s the object is at x=1.5 m. At t =1 s the object is at 2.0 m. What is the average velocity for the first interval (approximated at t=.25 s) and the second interval (approximated at t=.75 s)?   I'm mainly confused about what is meant by approximated at t=.25 and t=.75?

An object moves along a linear path with constant acceleration. Show mathematically that the average velocity of this object over any interval of time is equal to the instantaneous velocity at a point exactly halfway through that interval of time. Would this be true if the acceleration is not constant?

A car is crashed into a tree at the speed of 144 km/h. It took the car 4 seconds to stop. Assume that the car withstood the collision without substantial damage, and the driver received no more than superficial bruises to the surface of her body. (a) Draw a diagram that represent the collision and label all vector quantities. Make sure you pick axes and indicate directionality with signs and unit vectors. (b) Assuming that the acceleration was uniform during the collision, calculate the acceleration and indicate its direction on your diagram. (c) If (at best) a person can withstand 9g acceleration (9 × 9.8m/s ^2 = 88.2 m/s^2 ) only for a few seconds, is the driver of the car survived the crash?