Archive for May, 2011


Some things I’ve noticed…

  • Algebra is x sighting.
  • Rational people are partial to fractions.
  • Geometricians like angles… to a degree.
  • Vectors can be ‘arrowing.
  • Calculus teachers can go on and on about sequences.
  • Translations are shifty.
  • Complex numbers are unreal.
  • Most people’s feelings about integers are positive.
  • On average, people are mean.

May 31, 2011 at 1:17 pm 2 comments

Finger Multiplication

A digital computer is one who adds on his fingers, and this definition reminds me of a quote by Tom Lehrer.

Base eight is just like base ten, really… if you’re missing two fingers!

Many students know the trick for multiplying by 9 on their fingers, which I mentioned in yesterday’s post. However, most folks don’t know the following finger trick for multiplying two numbers that are greater than 5 but less than or equal to 10.

  1. On each hand, raise a number of fingers to indicate the difference between each factor and 5. For instance, 8 – 5 = 3 and 7 – 5 = 2, so to multiply 8 × 7, raise three fingers on the left hand and two fingers on the right hand, as shown below.

Finger Multiplication

  1. Count the total number of fingers that are raised on both hands, and append a 0. As shown above, there are 3 + 2 = 5 fingers raised, so this gives an intermediate result of 50.
  2. Count the number of fingers that are folded on each hand, and find their product. Again using the example above, there are two fingers folded on the left and three fingers folded on the right, and 3 × 2 = 6. 
  3. Add the results of Steps 2 and 3. For our example, 50 + 6 = 56, so 8 × 7 = 56.

We can use algebra to understand why this trick works.

Let L = the number of fingers raised on the left hand, and let R = the number of fingers raised on the right hand. Then the numbers we are multiplying are (5 + L) and (5 + R).

The total number of fingers raised is L + R. Consequently, the result from Step 2 is equal to 10(L + R).

The number of fingers not raised on the left hand is 5 – L, and the number of fingers not raised on the left hand is 5 – R. Their product, which is required in Step 3, is (5 – L)(5 – R) = 25 – 5(L + R) + LR.

Finally, combining them in Step 4 gives

10(L + R) + 25 – 5(L + R) + LR = 25 + 5(L + R) +LR = (5 + L)(5 + R),

which is the product we initially wanted to compute.

I recently learned one other method of multiplication. It does not require fingers, and I’m not even sure it’s useful, but it is interesting.

To find the product of two numbers, a and b, do the following:

Plot the points A(0, a), B(b, 0), and C(0, 1).

Draw segment CB, and then construct segment AD parallel to CB.

As a result, point D(d, 0) is a point on the horizontal axis such that a × b = d.

For instance, to compute 6 × 3, plot A(0, 6), B(3, 0), and C(0, 1). Then draw a line through A that is parallel to CB. This line will intersect the horizontal axis at the point D(18, 0), so 6 × 3 = 18.

Coordinate Graph Multiplication

May 28, 2011 at 10:31 am Leave a comment

A World Without 9

One of my favorite verses:

Once upon a time in our solar system,
We couldn’t make due without 9.
But Pluto’s not a planet now —
So 8 will due fine.

So sing the Barenaked Ladies in their wonderful song 7 8 9.

7 8 9

This got me to thinking: What would life be like if we didn’t have the number 9? It led to two questions that would be great fun to ask in a classroom:

  • Imagine if 7 really had eaten 9, and the digit 9 were removed completely from the number system. That is, not only is the number 9 gone, but you couldn’t make 19, 92, or 1,090,394, either. What would our number system look like?
  • Imagine if 7 had eaten only the number 9, but the digit 9 (or a suitable replacement) could still be used to make other numbers. That is, only the number between 8 and 10 is removed from the counting numbers. (Well, I guess technically we’d have to say that the entire interval [9, 10) is removed from the real number line, so that 8.999… = 10, but such a definition would go over the heads of most kids to whom I’d want to present this thought puzzle.) What would happen then? To think about it, calculate the results of the following problems:
    1. 8 + 7 =
    2. 8 + 14 =
    3. 12 + 22 =
    4. 4 × 5 =
    5. 6 × 10 =
    6. 11 × 12 =

As it turns out, my sons love this song. In fact, they love every song on Snacktime by BNL, and I have to admit that their daddy loves the album, too. Happily, my sons also love math, and they have shown a lot of interest in multiplication recently. A few months ago, Eli used the caculator to discover that 3 + 3 + 3 + 3 + 3 = 15, and he declared, “Daddy, if you add 3 five times, you get 15.”

“That’s right!” I said, and I explained that sometimes, instead of saying “add 3 five times,” we can just say “3 times 5.” Well, he thought this was extra cool. He entered 2 + 2 + 2 + 2 = 8 on the calculator and said, “Look, 2 times 4 is 8!” He has since converted thousands of addition problems into multiplication facts.

Just a few days ago, he made a statement that made my educator heart leap into my throat. He said, “Multiplication is just like adding a lot of times.” Of course, Keith Devlin might think this is a bad thing, but I thought it was a wonderful connection for Eli to make.

With their interest in multiplication sparked, we played with the Times Table on Illuminations. However, we did not use this as a modern-day set of flash cards with which my kids were supposed to memorize a bunch of unrelated facts. Instead, we talked about the patterns within the multiplication table, such as how the even numbers occur in the “times 2” row and that each number is found by adding 2 to the previous number. We also examined the square numbers, which occur along the diagonal.

Yesterday, I showed them the trick for multiplying by 9 on their fingers.

  1. Make two fists. From the left, count as many fingers as the number you want to multiply by 9, and raise that finger.
  2. Count the number of folded fingers to the left of the raised finger. This is the tens digit of the product.
  3. Count the number of folded fingers to the right of the raised finger. This is the units digit of the product.

The example below shows the result when multiplying 9 × 7. The seventh finger is raised. There are six folded fingers to the left of the raised finger, and there are three folded fingers to the right of the raised finger. Consequently, 9 × 7 = 63.

Hand Multiplication By 9

May 27, 2011 at 9:39 am 1 comment

Most Eggs-Cellent Math Jokes

You may have received the following advice about dividing fractions without thinking about it:

Ours is not to reason why; just invert and multiply.

Similarly, don’t waste your time trying to figure out why I’m posting a bunch of jokes about chickens and eggs. I can’t explain it. Just enjoy them, and please don’t analyze me.

How do you teach math to a chicken?
Show it lots of egg samples!

Why do chickens hate school?
They don’t like eggs-aminations!

Who tells the best math jokes on the farm?

How can you drop an egg six feet without breaking it?
Drop it from seven feet!

Why did the chicken go to school?
To get an egg-ucation!

Why do chicken coops have only two doors?
Because if they had four doors, they’d be sedans!

And a joke about the smartest chicken I know…

A chicken walks into a bar. “I’d like a burger and a beer,” he says to the bartender.

“Oh, my God!” the bartender says. “You can talk!”

“Well, look at that,” the chicken replies. “Your ears work!”

“But, you’re a chicken!” the bartender says.

“Ah, I see your eyes work, too,” the chicken says. “Now, can I have my burger and beer?”

“Certainly,” the bartender says. “Sorry about that. It’s just not every day that I see a talking chicken. What are you doing around here?”

“I’m working at the university,” the chicken says. He goes on to explain that he’s helping a professor with research on representation theory and integrable systems, but the bartender clearly has no idea what he’s talking about. So, the chicken enjoys his burger and beer and leaves.

A little while later, the owner of the circus comes into the bar. The bartender says, “You’re the owner of the circus, right? Well, have I got an act for you! I know this chicken who talks, reads, and drinks beer!”

“Sounds great!” says the circus owner. “Have him give me a call.”

The next day, the chicken returns to the bar. The bartender explains that he thinks he can get the chicken a great job at the circus.

“The circus?” asks the chicken. “You mean the place with the big tent, animals, lion tamers and trapeze artists?”

“Yeah!” says the bartender. “The owner would love to hire you!”

“Why?” asks the chicken. “What use would he have for an algebraist?”

May 24, 2011 at 12:38 pm 2 comments

Dissections are Fun, No Matter How You Slice It

Some folks have referred to me as “a real cut‑up.” Of course, those folks are generally septuagenarians who still use words coined in the mid‑1800’s. But I take it as a compliment, perhaps because I’m a fan of dissection problems.

Speaking of dissection, here’s one of my favorite quotes about jokes, which comes from E. B. White:

Humor can be dissected as a frog can — but the thing dies in the process, and the innards are discouraging to any but the pure scientific mind.

Like jokes and amphibians, geometric figures can be dissected, too. The following are three of my favorite dissection problems.

My all-time favorite is the Haberdasher Problem, originally proposed by H. E. Dudeney in 1902. It’s my favorite because of its simplicity — it can be described with just a few words, and it asks the would‑be solver to turn the simplest of all polygons, an equilateral triangle, into a square:

With three cuts, dissect an equilateral triangle so that the pieces can be rearranged to form a square.

The second isn’t really a problem to be solved. Instead, it’s a dissection that offers a surprising result. (At least, I was surprised the first time I saw it.)

Remove a tetrahedron of edge length 1 unit from each vertex of a larger tetrahedron with edge length 2 units. In total, remove four tetrahedra, one from each vertex. The image below shows one of the tetrahedra being removed. What is the shape of the solid that remains?

 Tetrahedron - Corner Removed

Finally, here’s an original dissection problem.

Form a pentagon by placing an isosceles triangle with height 1 unit on top of a square with side length 2 units. Then find a point P along the perimeter of the pentagon such that when P is connected to two vertices, the three pieces into which the pentagon is divided can be rearranged to form a square. Find the area of the largest piece. 

Pentagon - For Dissection

As usual, these problems are presented without answers or solutions. More fun can be had if you solve them yourself.

May 23, 2011 at 11:16 pm 1 comment

Let Me Translate For You

As any student of algebra will tell you, sometimes translating words to equations is harder than the actual math. For instance, the following limerick looks like a real bear:

A dozen, a gross, and a score,
Plus three times the square root of four,
Divided by seven,
Plus five times eleven,
Equals nine squared and not a bit more.

It’s a mouthful to read, but once you translate it to the equation below, you recognize that it is perfectly true.

Poem Equation

This poem was written by Leigh Mercer, a panhandling palindromist who earned money by drawing sidewalk caricatures. He is also the one who gave us the famous palidrome, “A man, a plan, a canal — Panama!” Although the poem above is often attributed to math textbook author John Saxon, it originally appeared in Games Magazine long before Saxon ever put it in a textbook. 

What can be more difficult than translating language to an equation is doing the opposite — taking an equation and figuring out the text from which it was derived. For instance, the equation 21x = 63 could correspond to the problem, “Walter Melon has 21 bags, each with the same number of apples, and all together he has 63 apples. How many apples are in each bag?” Or, it could correspond to, “How many dice do you have if there are a total of 63 pips?”

The following are some equations that were generated from quotations by famous people. Can you determine the original quotes and identify the authors?

  1. Humor = Tragedy + Time
  2. Success = Work + Play + Keeping Your Mouth Shut
  3. Necessities ≠ Wants
  4. lim (time → infinity) Lovetake = Lovemake
  5. Hesitation / Risk = Age
  6. Man = What He Has Done + What He Can Do + 0
  7. Results ∝ Effort
  8. Action = –Reaction
  9. Goals – Doubts = Reality
  10. Vlife ∝ Courage
  11. History = Σ(Things That Could Have Been Avoided)

 Original Quotes

  1. “Humor is tragedy plus time.”  – Mark Twain
  2. “If A equals success, then the formula is A equals X plus Y and Z, with X being work, Y play, and Z keeping your mouth shut.” – Albert Einstein
  3. “Our necessities never equal our wants.” – Benjamin Franklin
  4. “In the end, the love you take is equal to the love you make.” – Paul McCartney
  5. “Hesitation increases in relation to risk in equal proportion to age.” – Ernest Hemingway
  6. “A man is the sum of his actions, of what he has done, of what he can do — nothing else.” – John Galsworthy
  7. “The results you achieve will be in direct proportion to the effort you apply.” – Denis Waitley
  8. “Action and reaction are equal and opposite.” – Gertrude Stein
  9. “Your goals, minus your doubts, equal your reality.” – Ralph Marston
  10. “Life shrinks or expands in proportion to one’s courage.” – Anais Nin
  11. “History is the sum total of all things that could have been avoided.” – Konrad Adenauer

May 19, 2011 at 9:38 pm 3 comments

Mathy Names

Pink FlamingosWalking into the store was a large woman wearing flip flops, shorts that were too tight, and a shirt that revealed an inappropriate amount of her midriff. She approached the customer service desk, and she told the clerk that she would like to return the pink lawn flamingoes she had recently purchased. When asked the reason for the return, she responded, “My husband thinks they don’t look good next to the plastic deer and gnome on the lawn.” The clerk gave her a form to fill out. I watched over her shoulder as she scribbled her name:

Lois Carmen Denominator

That incident reminded me of my first teaching assignment. If name implies destiny, then I was primed for a spectacular year!

  • Matt Amatics
  • Cal Culator
  • Vin Culum
  • Cal Culus
  • Rose Curve
  • Polly Gon
  • Al Gorithm
  • Polly Hedron
  • Al Jabra
  • Ella Ment
  • Perry Meter
  • Polly Nomial
  • Hy Perbola
  • Lisa Perbound
  • M. T. Set
  • May Trix
  • Al T. Tude
  • Norm Ull

Know any others of this ilk? Post them in the comments section.

May 18, 2011 at 2:54 pm 9 comments

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About MJ4MF

The Math Jokes 4 Mathy Folks blog is an online extension to the book Math Jokes 4 Mathy Folks. The blog contains jokes submitted by readers, new jokes discovered by the author, details about speaking appearances and workshops, and other random bits of information that might be interesting to the strange folks who like math jokes.

MJ4MF (offline version)

Math Jokes 4 Mathy Folks is available from Amazon, Borders, Barnes & Noble, NCTM, Robert D. Reed Publishers, and other purveyors of exceptional literature.

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