You can't use rise/run with zero run,
but the derivative is only in terms
of h for h = 0.
It's like hooks, line, and sinkers,
it's a strange medical condition that
the howler troll gobbles them all the time,
X-ray shows it's mostly full of lead and spit.
Unfortunately it's taken this as also
recommending the (bad) diet to others.
Maybe this is what Burse kept mentioning about
"the idiot quack troll divides by zero wrongly".
The limit is the sum, in this sense:
the series that defines the limit is
no different than the sum, or the limit.
It's much better to approach various approaches
about finite differences and, minimization without
(standard, integral) calculus even, or the riddle
of the contour integral and line integrals generally
and defining rates and relations in rates over a
continuous time-like domain, constructively,
where then the real existence of infinitesimals
about other standard models of integers and for
example a standard complete ordered field for
real numbers, that the mathematics the troll
claims as booty really has its own home and place,
and, just because the troll sings its praises,
it can be confiscated.
Berkeley's treatise on infinitesimals, if you've
read it, details the features of standard ideas of
infinitesimals, and is more a lecture on rigor (and,
good-natured, at that), than actual denial.
Divide by zero.
Well-ordering the reals is a usual consideration in
modern mathematics after set theory, that many
intelligent people have followed as part of
usual mathematical curriculum, which can't lean
too much on just teaching simple tricks to avoid
analysis, that aren't general and are simple enough
as that instead there are calculators for them.
The derivative there is a function, defined as a
limit in terms of the differential, which is not a
finite difference, and is standardly no different
than zero, which is always different than the
denominator of any non-vertical line (which has none,
except for that one line to have rise/run = infinity,
in a convenient notation that isn't stupid though
is sharp, pointy, and dangerous for people who
don't know what they're doing or aren't fully
conscientious in obeying the laws of numbers.
Which is why it's kept usual simple and in
terms of the discrete and finite, as possible,
but that only continuous functions on continuous
domains have necessary properties for a mathematics,
not just a toy, or a trick, or a tool (or a turd).