Instantaneous Velocity
To find a slope of a graph at a point, this change of x has be zero, but
the fraction will be left undefined if the change of x is zero. This is
where the limit is used, when change of x will get smaller and smaller
that we can eventually find the limit of at which gives
us the derivative of the function.
Likewise, the formula for the velocity was. In a time-distance
graph, when change of time approaches zero, it will give the formula
for the instantaneous velocity after a certain distance it travelled.