Hello my mathematical genius friend. :)[should I edit that out? *blush*]

I have been sent the following mathematical joke of sorts. The person who sent it to me claims there are no flaws in it. But obviously there has to be a flaw, because the conclusion is incorrect. The problem is that I don't know how to explain the flaw---but I suspect it happens in that third line where it attempts to equate squared cents with squared dollars. Is there any way that you could explain the flaw in such a way that a seventeen year old Norwegian would understand? Don't worry, you don't have to say it in Norwegian, he speaks English.

If you don't mind having a look at this and explaining, I would be ever so obliged. And no pressure, but...pants may be on the line in this little bet I've entered into.

1. $1= 100¢ (so $0.1 = 10¢)

2. And, 100¢ = 10¢²

3. Then, 10¢² = $0.1²

4. $0.1² = $0.01

5. $0.01 = 1¢

The implied conclusion is

∴ 'a dime squared equals one penny'

Then we say 'Q.E.D.'.

Hm, if pants are on the line for my dear reader, I wonder what's on the line for moi-self (that is faux-French) (and 'faux' is French)? A review? Or two? Or three? Of my stories?

Let's leave my preening aside.

So, who sees the fallacies above that lead to the absurd conclusion?

If you do not see it, please think on this awhile before looking at the answer.

**The answer**

From basically the get-go this problem statement is erroneous and imprecise, but this comes from a fundamental laxity in understanding of what operators are and what operators do. Certainly, the first premise is correct: One hundred pennies does indeed equate to one dollar, for

1. 100¢ = $1

is a statement of fact about the conversion from one set of units (pennies or ¢) to another set (dollars or $). But already the trickster plays fast and loose, for indeed:

1. ... (so $0.1 = 10¢)

is still correct but the (implied) conclusion makes a statement about the square of dimes, not about the square of tenths of dollars.

Do you see the fallacy now?

No? Let's continue.

So 1. is true, insofar as I can throw it, and days where my back gives out (ah! me poor bones!) that's not very far, but it's far enough for this problem statement, so long as it goes no farther than that.

But it does. *sigh*

So now let's get into the lies:

2. 100¢ = (10¢)²[parentheses implied and erroneous]

This is a lie. It's a lie, lie, lie, lie, lie, lie, lie!

Huh?

The lie is this: a square of a thing is not the thing itself, and even if you know nothing about mathematics, you can prove this to yourself. Socrates did it with an unlettered and untutored slave boy, and you are further along than what Socrates had to work with.

So let's prove 2. false with an analogue.

Take a foot rule (sorry, my readers who do not follow the British Imperial system, which, oddly enough, includes Brits these days, too) and a large piece of butcher paper. Draw a line on the butcher paper measuring one foot.

a. _ = 1 foot.

Now, 'square' that line, by drawing three more lines to make a foot square on the butcher paper.

b. ❏ = 1 foot square.

So, is

c? 1 foot = 1 foot square

Obviously not! for that would be to say:

c? _ = ❏

Or, put another way, 'one thing of one thing is equivalent to one thing of entirely a different thing'. One gulp of water does not equal one gulp of bleach. One I wish to have with my breakfast, the other, I do not, as my father very unfortunately discovered the hard way one not-so-fine morning.

"But, but, but ..." you stutter angrily, "but isn't '10² = 100' a statement of fact?"

Yes, indeed, it is, but please remember '10 ≠ 10

*things*' is also a statement of fact. Number (with a capital 'N') is a class of classes as, e.g.,

*Introduction to Mathematical Philosophy*so clearly and succinctly explains.

The link goes right to the book, all 228 pages of it. It's a quick read, so please (re)read it.

So to state:

2. 100¢ = (10¢)²[parentheses implied and erroneous]

is to state a falsehood.

How do we correct it? Well, by replacing the (implied) error with an explicit (corrected) ordering:

2. 100¢ = 10²¢

Do you see the correction? It the former case, we erroneously squared the units along with the number, in the latter case we

*do not*square the units, we square the number solely.

Do I need to go any further? Or, do the fallacies fall out obviously in the rest of the assertions?

For completeness sake, I will review each step.

3. (10¢)² = ($0.1)²[parentheses implied and erroneous]

No. (10 pennies) squared does not equal (1 dime) squared.

But, a statement of fact of conversion is that (10 pennies) = (1 dime), but that's as far as it goes,

*and no farther.*

If we square pennies we have a new unit of measure called, I don't know: (¢²), and (¢²) does

*not*equal dimes. Not in this world.

4. ($0.1)² = $0.01[parentheses implied and erroneous]

Again, no.

Again: $(0.1²) = $0.01, but again, that is as far as you can go with that statement.

5. $0.01 = 1¢

is just a reformulation of the first statement and is true, yes, but redundant.

Remember from Frege's predicate calculus:

q |- p[read: 'q implied by p' or 'if p then q']

*[Shoot! why don't they have an 'implied by' HTML character?]*

But if:

¬p[read: 'not p' or 'p is not true (or provable)']

Then you can say anything you like for q, or, more correctly, you

*cannot*say anything

*at all*about q, because q does not depend on ¬p, it depends on p.

So, as it were: 'If I had done the laundry, we wouldn't have had this argument' and 'I didn't do the laundry' means I don't have a leg to stand on about why we had this argument, honey.

(Oops, sorry, ... but it's not like I have had this experience at all ...)

And, to the point of this article: 'If a (10 one-hundredths of a dollar) squared were to equal (10 pennies) squared, then ...'

Well, then anything, because '(10 one-hundredths of a dollar) squared = (10 pennies) squared' is false. So say away, because anything coming from a false premise is an absurd conclusion: whirled (black eyed) peas, butterflies flapping in the Amazon, and the Number 23. They may be true enough in their own right, but you can say

*nothing*about them from the false premise.

So, let's take the absurd conclusion:

∴ 'a dime squared equals one penny'

and reformulate it to be a true statement.

Well, the first thing we have to do it to get rid of the '∴', so let's do that, and then state the plain facts:

'a dime squared equals one penny' is an absurdity.

Q.E.D.

What can we take away from this?

Let's examine another absurity:

'Math is hard.'

No.

No, my dear ladies and gentlemen, 'Math' isn't 'hard.' Math is simple. Math can even be easy, for we learned from the Greeks, after all, that Math is one of the humanities. Math is simply a language. A language that can describe things exactly as they are and exactly as they are not. And precisely at that. It can even describe imprecision precisely. The 'hard'ness of mathematics comes from us, when we don't wish to be precise in what we are talking or thinking about.

Being precise ... well, that can be hard, I suppose, so then perhaps it's more precise not to say 'Math is hard' but to say 'Life is hard.'

Yes. That's true. 'Life is hard' as we choose to make it.

Oh, well. I never promised you a Rose Garden.

## 10 comments:

"3. (10¢)² = ($0.1)²"

I don't actually see anything wrong with this. It follows from 10¢ = $0.1. And if you expand it, you get:

100 square cents = 0.01 square dollars

What are square cents/dollars? Well, they're nothing anyone ever uses as a physical unit (like any number of other examples; minute^8000, perhaps; or squared apples). Notably with regard to the proof, a square cent is not a cent, and a square dollar is not a dollar, so this rule doesn't allow you to reason as the proposed argument does; it's true, but irrelevant.

I think that symbol you want is written as

→

in HTML. The escape code, I mean.

Yes, I thought the fallacy was about treating the units as part of the number.

Now, let me go and survey your stories. :o)

Sorry, the escape code was converted. :-D It is →

THANK YOU! Moving the dollars and cents outside of the implied parenthesis and the following line are what hit home with me:

'one thing of one thing is equivalent to one thing of entirely a different thing'

You laid this out so well and did such a nice job explaining it in layman's terms. And now if you'll excuse me, I've got to go collect a pair of Norwegian pants.

-HTHLiLa

P.S. You'll see that I also expressed my gratitude in the form of a review, Mr. Subtle.

Thanks for sharing your info. I really appreciate your efforts and I will be waiting for your further write ups thanks once again.

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