Pi Day Post on Irrational Numbers

March 14, 2009 by mathmom

My youngest son is obsessed with \pi.  “How much is \pi^\pi,” he asked me one day recently.  “I don’t know, a little more than 27,” was my unsatisfactory reply (which, it turns out, is only accurate for large values of “a little”).  Google Calculator tells me that it’s (approximately) 36.4621596 but what does it really mean to raise an irrational number to an irrational power?

Despite being inspired by my elementary-school-aged son, this post is not about elementary or  middle school math, in my usual style.  (For elementary Pi Day ideas, check out last year’s Pi Day post.)  But stick with me, if you will, and maybe you will learn something fascinating about irrational numbers.

Let’s step back a moment.  What does it mean for a number to be irrational?  My son knows that the decimal expansion of pi goes on forever and never repeats.  An irrational number is defined to be a number that cannot be expressed as a ratio or fraction (and a rational number is a number that can be expressed as a fraction).  All rational numbers have decimal expansions that are either terminating (such as 1/2) or repeating (such as 1/3).    Maria over at Homeschool Math has an excellent introduction to irrational numbers and proof that \sqrt{2} is irrational — the former should be accessible to middle schoolers, the latter high schoolers and beyond.   (Pi, as it turns out, is not only irrational but also transcendental, which means that it is not a solution to any polynomial with rational coefficients.)

Alexander Bogomolny at Cut the Knot offers an easy-to-understand proof that it is possible to raise an irrational number to an irrational power and obtain a rational result!  Many are familiar with Euler’s famous equation e^{i\pi} = -1 or equivalently e^{i\pi} +1 = 0.  Somehow one can raise an irrational number to an imaginary irrational power (!!!) and get an integer!  But what does it mean to raise a number to an irrational power??

Most of us have an intuition about what it means to raise a whole number to a whole number power.  3^7 means you multiply 3 by itself 7 times: 3\times3\times3\times3\times3\times3\times3.

What does it mean to take a number to a rational power? 3^{\frac{1}{2}} = \sqrt{3}, but why is this the case?  One intuition is to think that because it’s taken to the 1/2 power, you need 2 of them multiplied together just to get up to the original base, in this case, 3.  Or if you’re comfortable with the rules of exponents, you’ll know that 3^{\frac{1}{2}}\cdot 3^{\frac{1}{2}} = 3^{\frac{1}{2}+\frac{1}{2}} = 3^1 = 3.  So if 3^{\frac{1}{2}} times itself equals 3, it must be equal to \sqrt{3}  Similarly 5^{\frac{1}{3}} = \sqrt[3]{5}

But is there any intuition that can help us undertand what it means to take a number to an irrational power?  What could it mean to multiply \pi by itself \pi times?  The best explanation I can find is over at Ask Dr. Math, where Doctor Rob explains that we find the value of a number to an irrational exponent by raising the number to successively more precise rational approximations of the irrational exponent, and then using calculus to find where that sequence converges. (Although, that does not explicitly cover the case where the base is also irrational, the same general approach applies.)

Finally, Thomas Christie offers a nice explanation of Euler’s Equation.  (This link has been broken for the past few days, but I hope it will be back!)  In case Christie’s page doesn’t come back, you might wish to look here or here.

I hope you had fun exploring irrational numbers and powers with me today!  Now, go eat some pie. ;-) Happy Pi Day!

Happy Square Root Day

March 3, 2009 by mathmom

Square Root Day occurs only nine times per century, and one of them is today: 3/3/09.  You might have been busy celebrating New Year’s Day on the first Square Root Day of the century: 1/1/01, and you might have been distracted by Groundhog Day on 2/2/04, but make sure you don’t miss this one, or you’ll have to wait another 7 years until the next one on 4/4/16. :)

Since we won’t be in school for Pi Day (3/14), which falls on a Saturday this year, you might want to mention this one to your classes, if you read about it in time.

Happy Square Root Day!

Numbers with exactly four factors

February 25, 2009 by mathmom

Last night, around midnight, needing a lesson plan for my middle school group, I strolled over to MathNotations and clicked on his Number Theory category for ideas.  The Four Factors Problem filled the bill.  In this investigation, students are asked to find all the numbers less than 100 with exactly four factors, and describe the two categories of numbers that they find.

I decided that first we should find all the prime numbers less than 100 using a Sieve of Erastothenes.  Having the prime numbers in front of them is useful when trying to figure out how many factors a number has, since they might notice that 74 is 2 x 37 but then not immediately know if 37 can be factored further or not.  So, we spent the first 20 or so minutes of class crossing out multiples of all the primes up to and including 7 on a 100 chart.  We also figured out why we only had to go up to 7.

We counted 25 primes less than 100.  The kids noted that none of those numbers have exactly four factors, since primes have exactly two factors.  (Yay!)  One girl also pointed out that no perfect squares could have exactly four factors, because squares always have an odd number of factors. (Double Yay!!)  I asked the kids to each guess how many numbers we would find under 100 with exactly four factors.  Their guesses ranged from 14 to 25.  I also asked them to guess whether they would find more numbers less than 50 or greater than 50 with exactly four factors.  Everyone (myself included) thought we would find more such numbers below 50.  (Perhaps you would like to make your own guesses before reading further — I will post the actual number and distribution of the numbers at the end of this post. )

I had to clarify Dave’s “higher order” question: “These numbers fall into 2 categories.  Describe these categories.”   The first “guess” I got as to the categories were “odd and even”.  Well, yes, some of them are indeed odd, and some of them are even.  ;-)   So I clarified that I was looking for descriptions of two categories of numbers, all of which have exactly four factors, and that al numbers with exactly four factors must fal into one of the two categories.  I hinted that the categories have something to do with the prime factorizations of the numbers.

Most of the kids got most of the way through finding the numbers during class.  I assigned finding the rest of the numbers (after comparing my results with a few fast finishers, we told everyone how many numbers they were looking for) and coming up with the two categories.

We have in the past discussed how to find the number of factors a number has, based on its prime factorization.  (I can’t find a good page to link for that, so perhaps I’ll write it up myself at some point.)  I wonder if any of them will think about it in those terms.  I’ll report back to let you know how they did with that part of the challenge.

Related: Dr. Math page about finding numbers with a given number of factors

Answers:  There are 32 numbers less than 100 with exactly four factors.  And more than half of them are greater than 50!

While I wasn’t watching…

September 19, 2008 by mathmom

While I was off being distracted by life this summer, Kara Hazen of Hazen Happenings and Mathman at When Will I Use This? had nice things to say about my Blog.  Kara listed me among three “non-classmate” blogs she added to her RSS reader.  Mathman even gave me my very first blog award, which is only slightly diminished by the fact that he gave it to everyone on his blogroll. ;-)

Thanks for the kind words, folks!

I’ll have more content posted soon, I promise! :)

Why Math is Better than Politics…

September 16, 2008 by mathmom

“The best part about math is that, if you have the right answer and someone disagrees with you, it really is because they’re stupid.”

I found the above in an amusing list of quotes from an honors linear algebra class that was linked from Epsilonica, a blog I recently discovered, via his great post In Praise of Proving that Zero equals One.

Fill In the Number Puzzles

September 2, 2008 by mathmom

Danielle posted a question on my About page that I thought I’d answer in a separate post.  She wrote:

Hi MathMom!
I have a question… my daughter gets number triangles for math warm-up. E.g. Use each number only one time (1-9) and the sum of the numbers along each side of the triangle must equal 20.

She’s been doing them by trial and error, but that takes quite a long time the more complicated they get. Is there a formula or method that she’s missing?

Thank you for any help you can give her!

Danielle

Danielle,

There isn’t really a formula that can be used to solve generic puzzles like this, but the key to solving them is thinking about which numbers get used more than once.  In a triangle puzzle, this is the numbers in the corners.  Each of these numbers gets included in 2 of the sums, while the rest of the numbers get included only once.

In the example you gave above, your daughter should first think about what the sum of all the available numbers is if they were each used only once: 1+2+…+8+9 = 45.  Since we need the total of the 3 sides to be 60 (3×20) we know that the numbers that are used twice must add up to 15 (the difference between 60 and the 45 we’d have if each number were used just once).  So, the corners of the triangle should add up to 15.  Unfortunately, there are a lot of ways of making 15 out of 3 of the numbers from 1 to 9.  But it does cut down the trial and error a fair bit to start.  I tried a few combinations and was able to find answers pretty quickly using that constraint.  I hope this insight makes these puzzles a little more fun and less frustrating for your daughter, Danielle!

Here’s another type of fill-in puzzle.  You don’t use sums to solve it, but the key to it rests in realizing which spots are “special”.  I’ll leave it at that for now so as not to give it away for those who’d like to try to solve it.

I’m still out here…

September 2, 2008 by mathmom

In case anyone was wondering, I’m still here.  Real life intervened over the summer and distracted me from blog reading and writing.  But now I’m preparing to go back to my volunteer role in teaching math problem solving at my kids’ school, and I’m excited about that.  Hopefully I’ll find time to write about it as I go.

More on Math Education

April 29, 2008 by mathmom

I found this forum due to the fact that someone there linked to my last blog post. (The link is on page 10 of the discussion if you want to check out the context.) There’s a great discussion there about some non-orthodox math education options (mainly, I believe, intended for homeschoolers or parents of gifted kids wanting to supplement at home).

The thread was started by a parent who had just heard a presentation by a local mathematician who advocated that kids shouldn’t learn math using a textbook at all — it should all emerge naturally from various scientific/engineering explorations. Although I don’t particularly agree with that idea (not least because “natural” situations tend to involve ugly numbers, which make hand computation impractical, and argue for the use of calculators — I’d be in favor of calculator use in such cases, but not if it’s the only exposure to computation the student is going to get!), the thread contains a lot of good ideas about math education, many relevant to both gifted and non-gifted students, and many as relevant to classroom instruction as to homeschoolers.

On page 11 of this discussion, there is a link to a great-looking free online math course intended for “adult learners and high school teachers”. NOT a course in computation or algebra or trig, but a course in cool math topics, such as prime numbers, combinatorics and game theory. Although they are aiming it at HS teachers, I think it could be a great resource for math-phobic elementary teachers, not because they would necessarily pass on that particular content to the kids they teach, but hopefully to help improve their attitude toward math, so that they could pass on some excitement about it (and also give age-appropriate introductions to many of the topics, even at the elementary level).

I haven’t had a chance to actually go beyond the overview of the course — perhaps I will have a chance to review it more completely at a later date. But it definitely seems worth a look.

Math Wars

April 27, 2008 by mathmom

Over at MathNotations, there is yet another raging debate about math education. I wasn’t going to get involved, but I finally broke down. ;-) I started this as a comment on that post, but it is so long that I decided to post it as a blog post here.

So… here are some of my thoughts on the issue of math education, particularly for elementary and middle school kids. Let me say right up front that I am not arguing for or against any particular math curriculum here. My kids and I do not have extensive experience with any particular math curriculum. My own kids are not taught math from a single curriculum, but are rather taught using a wide variety of materials, exercises, explorations, etc. I have friends who are very pleased with Singapore Math. (I have used some of the workbooks for my younger sons as well, just for fun.) I have other friends whose school does an awesome job with Everyday Math, though I realize that that appears to be the exception and not the rule with that curriculum. What is key is great teachers, in either case. But that is all I am saying about particular curricula.

There is a problem that great teachers like Dave and Jonathan seem reluctant to admit, and that is that many otherwise great elementary teachers are poor math teachers. Elementary teachers are generalists, and for many of them, math is something to “get through” and to get kids through. It is not something they ever enjoyed, not something they are comfortable with, not something they are good at teaching. These teachers destroy curricula like Everyday Math that really require a great teacher. They would probably also be poor at teaching Singapore Math. So… as much as great teachers should not be restricted, or heaven forbid scripted, in the way they teach math, we have to do something about the fact that many elementary kids are getting their fundamentals from instructors who hate math and who are no good at teaching it. Some people think the answer is a highly scripted curriculum. I think that sounds horrible! I think the solution probabaly requires thinking outside of the box a bit more. Perhaps even elementary students need to have specialist math teachers, just as they often have specialist art and music teachers. (I’m focusing on elementary here because this is less of a problem in middle and high school where math teachers are generally specialists. There are better teachers and there are worse teachers, but few middle school, high school or college math teachers hate math.)

Steve H is concerned about kids who don’t have mastery of basic number facts (addition, multiplication, etc.) at an age where he thinks they should. He blames Everyday Math for the fact that kids in his child’s school don’t know their facts, but I’ve never seen any curriculum that builds this drill and practice in. In my experience, this has always been done in addition to following whatever “curriculum” the school is following, and I would think it could be done just as easily in conjunction with Everyday Math as it could with Singapore Math or any other curriculum. It also generally happens mostly at home. Teachers must expect students to study their facts at home, and must assess their progress, but parents must get out the flashcards, or get their kids onto a practice game or website, and get their kids to practice at home.

Class time should not be taken for memorizing number facts, no matter what curriculum you’re using. A few “mad minutes” a week are enough to assess how that is going and keep the kids motivated, and only takes a few minutes out of the math instructional time.
There is an argument that states that mastery of number facts and procedures (such as long division), performed accurately but without necessarily any understanding of why they work, is the most critical job of any math curriculum, and should be addressed before taking on anything else. These are certainly basic skills that all kids should learn. But to be honest, as much as I hate calculator use in school, in this age of calculators and computers, efficiency at hand computation is not, IMO, the most critical math skill for kids to learn. I am NOT saying that it should be ignored, or that kids should be allowed to skip it, and just use calculators in class (see rant linked above). But it is not, IMO, the be all and end all of math education, nor is it a prerequisite, IMO, for studying anything else.

What I consider even more important is a strong sense of number. I want kids who know immediately when the answer they got (either by hand computation or with a calculator) is way off. I want kids who have an instinctive understanding of the distributive law before it is ever formally taught or named (12 sevens is obviously the same as 10 sevens and 2 more sevens). I want kids who know when the amount of change handed to them makes no sense. I would rather have a kid who can multiply 64 x 25 mentally (by halving 64 twice and doubling 25 twice, to see that it’s equal to 16 x 100 = 1600) than a kid who can sit down and carry out the long multiplication with pencil and paper, by rote.

You can’t just sit down and teach kids “number sense”. Certain mental math tricks can be taught and practiced, but the way to achieve real numeracy involves lots of experience playing with numbers, with manipulatives, with measurements, etc. (Ok, I said I wasn’t going to talk about curricula, but I will say that from what I’ve seen of Singapore Math, it seems to do very well at guiding kids toward the development of good number sense.)

“Mastery” is also a slippery concept. I’ve seen kids “master” skills and then promptly forget them. I’ve seen kids who can easily do a page of long division quickly and without errors, who 6 months later will have no clue how to do long division… This is part of the reason why schools often utilize spiral curricula. Because if you have gifted kids, it’s easy to think “master one skill and then move on to the next” is the only sensible way to approach mathematics education. But most kids need more repetition than that, and many kids don’t fully “get” something the first time, even if they appear to have “mastered” it. Steve is right that a spiral curriculum can lead to a lax attitude of “it’s ok if they don’t master this now, because they’ll see it again later” that goes on ad infinitum, and the kid never masters anything. This is clearly no good. But the solution isn’t necessarily to take away the spiraling for those who need it, IMO. The solution is to have limits — for example, it’s ok if they don’t completely “get” long multiplication when it’s previewed in 3rd grade, or even when it’s introduced more formally in 4th, but they have to get it when it’s reviewed in 5th, or they shouldn’t move on. It needs to be clear where in the spiral one is, and whether this is a preview, or core instruction, or a last chance review, and make sure that kids really “get it” before moving past that “last chance”.

Dave is advocating and providing samples of “non-routine” problems. These often considered the domain of math contests, and to be reserved for only the most gifted math students. In my opinion (and I know Dave agrees), this type of problem solving is important for math students of all levels of ability for many reasons.

First, it provides a fabulous way of helping students to appreciate the uses of the procedures and skills they have learned or are learning (or in some cases, motivates a procedure yet to be learned). It provides a great way for teachers to re-assess students’ continued mastery of multiple skills, to see which need further review or clarification. This addresses that slippery slope of mastery. Invariably, some previously “mastered” skills are shown to be weak, and must be re-visited. In this way, we avoid just “checking off” topics, and make sure that students can recognize when a particular method or procedure is called for, and
remember how to use it long after the first time that they supposedly “mastered” it.

Second, this is the kind of thing that “real mathematicians” do! If one of the goals of K-12 math education is to prime future mathematicians, this is a valuable opportunity to do so. A “mathematician” does not sit down and solve 25 ratio and percent word problems, knowing exactly which skills are required to perform the computations. Instead, she investigates “puzzles”, looks for interesting patterns makes new discoveries, generalizes results. For students who might have the inclination to pursue mathematics further at some point, this kind of experience early on may spark their interest, and excite and motivate them.

Third, it develops self esteem and confidence. This may seem surprising, since the problems are very hard for most students. But the students that I work with know that the problems I bring them are meant to be hard. That they aren’t meant to be able to solve them all on their first try. That they may need help. But, when they do solve one correctly on their own, they are so very proud of themselves, and rightly so. A student gains so much more self-esteem and confidence from struggling and succeeding at something hard than they do from breezing through something easy. (Not to say that the rest of the math curriculum is easy, of course, but there is a persistent fallacy that I’ve seen many times, that the way to develop self-esteem in kids is to make sure to give them lots of easy work that they can effortlessly succeed at.)

Fourth, it builds transferrable problem solving skills. Math class is not the only place where people will have to solve difficult problems, problems where the best approach isn’t obvious at first glance. Practice in solving problems like this transfers to many different areas of the curriculum, and to “real life” as well.

Personally, I’d love to see what Dave would come up with as a Math Curriculum. However, I don’t think it’s so easy to just write a great curriculum and have the world beat a path to your door. There are huge corporations with a lot invested in selling math curricula, that he’d be competing with.

Skip-Counting for Numeracy

April 11, 2008 by mathmom

Most kids learn, in kindergarten or first grade, to count by 2’s, 5’s, 10’s. By rote. This has its uses, but going beyond that level of skip-counting can make great numeracy practice, and it seems to be quite under-used.

In our school, we have the primary kids skip-count by 2’s starting from numbers other than 2. Then they practice skip-counting by other numbers, again starting at arbitrary places. Forward and eventually backward. This is an easy one to differentiate, because you can have some kids counting up by 2’s and others counting backward by 7’s at the same time. :)   Eventually skip counting is used to introduce multiplication.

Today I asked the intermediate group (approx 3rd – 5th grade math levels) to do some skip-counting for me, forward and backward by 2, 3, 5, 10, 20, 25, 50, 100 and 99! Boy that last one was a doozie! (“Can we use calculators?”) But all of it was clearly good practice even for these “big kids”. For counting (up) by 99’s, they quickly realized that they could add 100 and then subtract one. But a LOT of them got stuck crossing the 1000 boundary (going from something like 987 to 1086). I didn’t think of it at the time, but now I think that pointing out that 1000 is really the same as “ten hundred” might be helpful for a lot of them. Now backward by 99’s was really tricky! So… you first subtract 100 and then… what? Only one of the 6 kids I was working with today (only half of the usual group) really understood that you would then have to ADD 1 back. “I’m taking away a little LESS than 100, so should the answer be a little more or a little less than what we got by taking away a whole hundred?” This is not intuitive for a lot of kids!

So, if you’re working with elementary school aged kids, skip count! Regularly! I think it’s a key skill for developing number sense.