Abstract:Instantaneousness
which has not length could not compose the time segment possessed length.
Instantaneous velocity is approximate value of average velocity at sufficiently
small time segment. Use omnipotent approximation derivative, the necessary and
sufficient condition reached extreme value of function could be obtained. Use
approximation derivative can be explained meaning of instantaneous velocity
without contradiction. Regard differential as variable, can explain properly
the application of differential and differential of function. The expression to
calculate error of indirect measure in text-book is not reliable.