1. Instantaneous speed is the speed of an object at a specific point in time, whereas average speed is the total distance traveled divided by the total time taken. In other words, instantaneous speed is a single value that describes an object's speed at a particular moment, while average speed is a measure of how fast an object is moving over a period of time.
2. The equation used for calculating average speed when an object is uniformly accelerating is
v_avg = (v0 + v1) / 2
where v_avg is the average speed, v0 is the initial velocity, and v1 is the final velocity.
It must be noted that this equation is only accurate when the acceleration is uniform, and the time taken for the change of speed is small. When the acceleration is not constant or the time is long, the equation for average speed is different and more complicated.