### blogging calc i

without wanting to commit myself… here goes.

i’m *undercommitted* this quarter goodness knows.

my calculus blogging from spring ’09

might come in handy (but the Calc III stuff

is mixed in with the Calc I).

for that matter, the common errors page (not by me)

that i cited yesterday is *bound* to come in handy.

heck. learning math on the web?

just like learning anything else,

there’ll be plenty of good info… more than enough

to build a course around…

in the ever-amazing wikipedia. let’s see.

consider this list of calculus topics.

hmmm. it doesn’t refer, specifically and directly,

to the topics we looked at on tuesday…

exponential functions, inverse functions, log functions…

but *does* link to the precalculus page that

*does* treat of these topics specifically (though not directly;

instead it provides links to w’edia pages on each).

continuing in this vein, i’ve just looked (for the first time)

at the exponential function page. hmm.

i imagine myself a beginning student. what do i see?

forest-and-trees issues abounding… there’s an *awful lot*

of material here! but maybe it’s clearer than the textbook

even so. in particular, the article is (very rightly) about

*the* exponential; of course i mean the one with base “e”.

and, right out front, in the first sentence in fact, they’ve got

“the function e^x is its own derivative”.

every calculus *teacher* understands, at least to some extent,

that the importance of the number “e” is very closely tied to

this property… it’s the base that makes exponentiation

“work out conveniently” in “doing calculus”.

most of ’em, if pressed, would probably be able to tell you

that “y = e^x” is the only non-trivial solution

to… the world’s simplest interesting differential equation…

. (the trivial one is y’ = 0).

every calculus *book* obscures this point to some extent.

and there are reasons. one has not *defined* , after all.

*our* text—”stewart”—follows the usual pattern

of “discuss exponentials generally first”

(y = A^x for A a positive

number different from 1) and then singles out the case A=e

as the one having a tangent of slope 1 at its y-intercept.

this can hardly be very motivating for a beginner.

and, anyhow, neither has “tangent” (to a curve at a point)

been defined… so (as far as i can see) *nothing* is gained

in terms of “formal correctness” by focusing on this

particular *detail* of the fact that the exponential

is its own derivative.

okay. there are *better* reasons. textbooks *should*

review exponential-functions-generally (and provide

lots of practice problems). nobody’s going to

understand very much about y=e^x that doesn’t

know anything about its first cousin y=3^x.

still it seems to me that

somewhere pretty close to the moment that

the hugely-important constant “e” is introduced,

it would be helpful to at least some students…

students like i imagine myself to have been,

for example… to have some *succinct*

and *correct* justification (even if its details

can’t be spelled out fully with the concepts

already covered in the prerequisites-so-far).

if i recall correctly, my own experience of

learning-about-e was rather a horrible mishmosh

of formal-correctness and we’ll-learn-about-this-later.

i *did* learn about it later but it was an accident of history;

if i’d merely been a math *major*, it’d’ve been obscure

to me all my life, but since i went on to be a

*graduate student* in maths, i eventually

considered myself duty-bound to make sense

of it all (and had the “mathematical maturity”

to do ahead and do it).

it makes good sense *formally* to consider

the natural-logarithm function (“ln”)… defined

(of course!) as the integral-of-the-reciprocal…

*first*, and define “e” as the solution to

ln(x) =1.

hey, madeline just woke up. more later.

March 31, 2011 at 7:34 pm

When I teach calc I (it’s been a while!), I work through the derivative of 2^x with them. We have a mystery number as part of the derivative (this number is the slope at x=0). Once in a while, someone recognizes .69, but mostly we just keep it around for a bit. I have them do 3^x, and we get a new mystery number, 1.1. We get if y = a^x, then y’ = k*a^x, and k increases as a gets bigger. So we invent e, and then we can find ln2 and ln3 from that, and clean up the others.

March 31, 2011 at 10:36 pm

somebody *recognizes* .69

(as an approximation to ln(2))?

wow. *i* didn’t recognize it!

anyhow, this certainly looks like

a reasonable way to motivate the

“exponential whose slope at (0,1)

is 1” approach to defining the

natural-exponential function.

*if* we had world enough and time.

at least an hour lecture on this one topic.

i’ve seen this approach in texts,

of course. never had a chance

to do the lecture this way though.

if there’s an intro-calc course

with this kind of leisurely pace,

*i’ve* sure never seen it.

many of the students are going to believe

that the whole “derivatives are limits

of difference quotients” thing is just

a bunch of mumbo-jumbo anyway.

“just show me the formulas;

never mind why”.

if i were to design the course, everybody

would darn well have to find derivatives

by taking limits-of-DQs even if that meant

taking two weeks to do linear functions.

of *course* students who don’t know

how and why to “cancel” the delta-x’s

and when to quit writing “lim”

for *easy* functions aren’t somehow

suddenly going to catch on by watching

it done with *tricky* functions…

but, hey. look here. that section was

“covered” last week. too late! too bad!

better just “memorize” something and pray.

March 31, 2011 at 10:47 pm

Maybe my memory’s bad – maybe no one does ever recognize it, I’m not sure…

Our course is definitely not leisurely, but this seems an important point, and I take the time. I go too fast over lots of other stuff. (A large component of the art of teaching well within a school situation may be knowing when to slow down and when to speed up.)

In my intermediate algebra class, I tried to go too fast through the rational expressions and roots, so I’d have more time for my beloved exponential functions. But the students asked lots of good questions and slowed me down. We’ll still do the murder mystery in the exponents unit, but something will have to give at the end. I may skip inequalities entirely (along with conics and series).

April 2, 2011 at 12:56 pm

i quite agree about the “large component”.

the selection of topics for our courses,

and their textbook presentations,

are determined by committees

(and corporate entities even less

interested in human needs).

“historical forces”… inertia mostly…

play a large part too.

evolution… *not* intelligent design.

anyhow, after one has presented

a course by following the stated rules,

one will have learned quite a bit

about where the most useless timesinks

are and what essential material is

being mentioned and passed over

much too quickly (or ignored altogether).

if i’m right in thinking that my students

are generally lucky to have me as a teacher,

my ability… and willingness… to use my

judgement on these matters might very well

be the best single reason.

some of the other teachers aren’t able;

i hate dissing math teachers, so enough

about that. many, maybe most, of the rest

aren’t *willing*… and no damn wonder.

you’ll be punished if you try.

much safer to just say “hey, i’m doing

my job” like any other government drone

refusing to help the people they’re

supposedly serving because the important

thing is to appear to be following rules.

enough. thinking about working conditions

instead of actually working has made me

a nervous wreck. (that and things like

“no paychecks this quarter”.)

i’m hoping it’s the *graphical* component

of “inequalities” you skip. everybody needs

to know *something* about ’em…

namely “change signs and directions

together or not at all”.

then they throw in absolute values

and botch the job badly; along comes

*graphing* inequalities and they botch

the job even worse.

the final exam at my ex-community-college

gigs had a “graph the inequalities” problem

wrongly solved on the answer key

for years. probably still does.

and no being with the authority to fix it

has any interest in the truth.

conics for precalc is also well worth skipping

(if only one could). i first saw ’em in calc ii

and got nothing out of it but confusion.

too bad, too. if geometry hadn’t been

sidelined somehow this could be great stuff.

conics are closely related to all this drawing

i’ve been doing. but the very math *majors*

typically learn nothing or almost nothing

about projective spaces alas. and waving one’s

hands at the topic in the form of “memorize

a whole bunch of formulas you’ll never need

again” would obviously be a bad idea if one

were actually to work with a few classes

that’ve been made to try it this way.

series? not sure what you’re getting at.

sometimes… business calc, e.g. …

one sees “sigma” notation introduced

in contexts where about three weeks

worth of material is to be presented

in a day. my recent “discrete” class

did this too. one can do the lecture

of course but *only* students having

considerable prior exposure are likely

to get anything *out* of ’em.

somewhere along the line, though…

calc iii or so typically… they’ll become

essential.

April 3, 2011 at 2:14 am

What I’d have done with series in intermediate algebra is show them how mortgage payments are calculated, and how interest accumulates. I convinced students back in Michigan to never take out 30 year mortgages.

We hit very basic inequalities as we talk about domain, and I’ll be using that as my context if we have time for our inequalities unit.

April 3, 2011 at 4:05 pm

good discussion for me to read! Thanks and thanks.

Jonathan