a) if along a straight line, then is . But if and [ and leads to similar conclusions...] is and in 'proximity' of is so that the limit is 0. If and is and the limit is again 0...

b) as seen in a) there is almost one 'trajectory' for which the limit is 0. Now we consider the 'trajectory' defined by we find that the limit is 1, so that no limit exists...

Kind regards