Hello, I have a question regarding the first equation above.
it says dui=ai*dr=ai*aj*duj but I wonder how. (sorry I omitted vector notation because I don't know how to put them on)
if dui=ai*dr=ai*aj*duj is true, then
where lim |dr|,|duj|->0
which means rhat=ajhat.
are both the directions actually the same or do we just don't care about directions because both will eventually be small increment of each component?