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### 'Why' Questions in Math

```Date: 04/19/2002 at 13:09:14
From: Sharyn
Subject: "Why" Questions in Math

In science, when one asks why, there seems to be a "real" (to me)
answer. Such as chemistry, where one can identify a chain of concrete
events that leads to "now."  But if one asks, when calculating the
standard deviation, why the sum of deviations around the mean equal
zero, the answer seems to be a long "proof" that I won't understand.
(It seems the answer is that someone just discovered that, so we
square the values to add them.)

I always thought algebra was mystical and there was a whole lot I did
not "get" (and everybody else did "get").  If I had known to just
memorize things, I bet I would have been a lot better off. (Maybe math
teachers should say something about that?)

Sharyn
```

```
Date: 04/19/2002 at 14:32:02
From: Doctor Ian
Subject: Re: "Why" Questions in Math

Hi Sharyn,

In science, (we assume that) there is a set of rules (the fundamental
laws of nature) in operation, and the task is to figure out what the
rules are by observing the results that occur when the rules are
followed. Basically, it's an attempt to reverse-engineer the machinery
of the universe.

In math, it's the other way around - we get to choose the rules, and
the task is to discover the results of choosing any particular set of
rules.

There is a superficial similarity, which leads some people to confuse
the two pursuits. In science, the way you test a theory is to codify
it as a set of rules, and then explore the consequences of those rules
- in effect, to predict what would happen if those rules were true.
You do the same thing in math - and in fact, the way it's done in math
serves as a model for the way it's done in science.

But here is the big difference: In science, as soon as your
predictions conflict with experimental data, you're done. You know
that your rules are wrong, and you need to start putting together a
new set. In math, this kind of conflict can't arise, because there is
no necessary connection between any mathematical theory and the world.
The way you 'test' a set of rules in math is to see whether the
results they produce are interesting enough to induce mathematicians
to keep playing with them.

We might summarize the situation this way: Science is the pursuit of
_the_ correct description of _this_ particular world; whereas math is
the pursuit of interesting descriptions of possible worlds. Whereas
scientific theories are right or wrong, mathematical 'theories' are
merely interesting or uninteresting.

To get back to your question, there is nothing wrong with asking 'why'
questions in math! But it helps if you know ahead of time that
ultimately, the answer will always be 'because we decided to use these

However, there are lots of levels of 'why' explanations before you get
to that point. To take your example of the reason that the deviations
from the mean add up to zero, that's a consequence of the way that the
mean is defined.

Whether the explanation is long and complicated or short and intuitive
depends on whom you're trying to convince. Here's one way to think
about it. To find the mean of a bunch of values, we add up the values
and divide by the number of values:

v1 + v2 + ... + vn
mean = ------------------
n

Does that make sense so far?  Okay, now suppose we've found the mean,
and we subtract that from each of the values:

(v1 - mean) + (v2 - mean) + ... + (vn - mean)
? = ---------------------------------------------
n

(v1 + v2 + ... + vn) - (mean + mean + ... + mean)
= -------------------------------------------------
n

(v1 + v2 + ... + vn)   (mean + mean + ... + mean)
= -------------------- - --------------------------
n                           n

(v1 + v2 + ... + vn)   n * mean
= -------------------- - --------
n                n

Now, the first term is just the original mean. And the second term is
also the original mean. So the right side has to equal zero.

What does this tell us? It says that if we subtract the mean from each
value, and then add the resulting values, we end up with zero. But
this is just another way of saying that the deviations from the mean

Now, if you think of the mean as an arbitrary formula that you
evaluate on a bunch of numbers, then this is indeed sort of
mysterious. Why does it work out this way? Is it magic?

Well, just the opposite, in fact. A better way to think of the mean is
this: If you have a bunch of values, the mean _is_ the value that you
would have to subtract from all of them to get the sum to add up to
zero. That's what it _means_ for something to be a 'mean'.

Do you see the difference? It's not that we make up the formula and
then this weird consequence drops out. It's that we want to get the
consequence, and we look around for a formula that will give it to us.
The problem, of course, is that it frequently gets _taught_ in the
opposite direction. Hence the kind of confusion that you've been
experiencing.

For what it's worth, memorizing things is just about the _worst_
possible way to go about learning math. If you have to memorize
something, it usually means that you don't understand it. And if you
don't understand it, your chances of being able to apply it in
unfamiliar situations are slim indeed.

In the case of the formula for the mean, if you understand what a mean
is supposed to do - that it's supposed to distribute the values in a
certain way around a central value - then you can actually figure out
what the formula would be (by working out some simple examples), if
you happen to forget it.  This is the case with an awful lot of math.

To give you another example, I can never remember what the formula for
the surface area of a cylinder is.  But I know that it's made up of
three parts:  two circles, and a side.

And I know that if I slit the side open and unroll it, it's actually a
rectangle. Now, I _have_ memorized the formulas for the area of a
circle,

and for the area of a rectangle,

area = width * height

and the area of a triangle,

area = (1/2) * base * height

but those are the _only_ formulas for areas that I've memorized.
Everything else, I work out in the way that I just showed you.

Similarly, I can never remember what

b c
(a )

is supposed to be.  But I know what an exponent is, so I can use
a simple example, like

2 3
(a )  = (a*a) * (a*a) * (a*a)

6
= a

2*3
= a

This approach doesn't really take any longer, and this way I know that
I haven't mixed up something in my memory.

or anything else.

- Doctor Ian, The Math Forum
http://mathforum.org/dr.math/
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