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Algebra ManipulativesComparison and History by Henri Picciotto | |
If you are not familiar with the basics of algebra manipulatives, I recommend you look at other articles on this site first, as this one is rather technical. | |
Article outline |
Even though they cannot make algebra easy, manipulatives can play an important role in the transition to a new algebra course:
There are four main commercial versions of algebra manipulatives. In order of their appearance on the market, they are Algebra Tiles (Cuisenaire), the Lab Gear (Creative Publications, now McGraw-Hill), Algeblocks (Southwestern Publishing), and Algebra Models (Classroom Products). All four provide a worthwhile model of the distributive law. However, note that only the Lab Gear and Algeblocks allow work in three dimensions.
The Lab Gear
My creation! Read an introduction in:
Then, see the animated graphical mini-intro to the Lab Gear by George Collison of INTEC, and the applet representing the factoring of a trinomial with leading coefficient 1.
For a full presentation of the Lab Gear, see my algebra books, or the video course I helped write for Dr. Ed Dickey of the University of South Carolina. This article is adapted in part from the print material that accompanied the video. (How to get some of my publications.)
The Algebra Tiles
The Algebra Tiles are inexpensive and widespread. They make it possible to do most of the activities that are needed to introduce and explain the distributive law and factoring.
Algeblocks
Algeblocks do have a variation on the corner piece (the quadrant mat) and they do support two variables and three dimensions. They lack the 5, 25, 5x, and 5y blocks, which is merely inconvenient. You can use them to do almost all the basic activities about the distributive law and factoring.
Algebra Models
Algebra Models do include two variables, a 5 block, and a variation on the corner piece (the Work Tray.) They are tiles, and thus they are less expensive, but cannot be used in three dimensions. Their main advantage is that alone among available algebra manipulatives, they are incorporated into an excellent Algebra text: Algebra Connections, by the College Preparatory Mathematics program (CPM).
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The representation of minus is the most controversial part of manipulative models of polynomial algebra. The four commercially available products each handle it in a different way. These ways have consequences in four areas:
The Algebra Tiles model of minus is based on color: one color corresponds to positive numbers, and another to negative numbers. This is then generalized to variables: blue x's represent x, and black x's represent -x.
The Algeblocks model of minus is based on position: blocks in the shaded area of the various Algeblocks mats are taken to be preceded by a minus.
The Lab Gear model involves two different representations, both based on position: blocks in the minus area of the workmat, and blocks sitting on top of other blocks ("upstairs") are taken to be preceded by a minus.
The Algebra Models model is based on color. However, at least in the CPM implementation, this is combined with a workmat-like minus area, which makes it possible to do much work with reading and simplifying expressions, equation-solving, and the like -- very much like what can be done with the Lab Gear. Moreover, CPM avoids using minus in the area model of multiplication, and thus the geometric integrity of the model is not undermined. (See next section.)
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With the Algebra Tiles, to multiply (y+3)(y-2), you do the same thing that you would do with (y+3)(y+2), except that you use the "minus" colored tiles for -2, and for -2y and -6 in the product.
Similarly for (y-3)(y-2), you do the same thing that you would do with (y+3)(y+2), except that you use the "minus" colored blocks for -2, and for -2y and -3y in the product.
Note that in this model, the products (y+3)(y+2), (y+3)(y-2), (y-3)(y+2), and (y-3)(y-2), are all congruent, which is geometrically incorrect, since for example y+3 clearly should be longer than y-3.
The Algebra Models have exactly the same problem.
With Algeblocks, factor blocks are placed on the right or above the center of the quadrant mat if they are preceded by a plus, and to the left or below, if they are preceded by a minus. The resulting product subrectangles are considered to be preceded by a plus for the 1st and 3rd quadrant, or a minus for the 2nd and 4th quadrant.
In this model also, the area model loses its geometric integrity when minus is used:
With the Lab Gear, the upstairs representation of minus accurately represents y-2 as 2 units shorter than y. It is then possible to multiply in the corner piece, and use upstairs blocks in the product, to obtain rectangles with geometrically correct dimensions. However how to do this is not easy to learn in a case such as (y-3)(y-2), and many Lab Gear users stop short of teaching this to their students. This is not a big problem, because the Lab Gear materials encourage a transition from blocks to symbols with the help of the "multiplication table" format for polynomial multiplication. That format is visually related to the basic Lab Gear multiplication format, it works fine with minus, and it guarantees that the student does not remain dependent on the blocks.
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I encourage you to compare the curriculum materials provided with the four models. In my opinion, the Algebra Tiles manual is confusing and essentially unusable. There is much of value in the Algeblocks binder, which seems in large part inspired by the original Lab Gear books. Moreover, their ingenious (if mathematically flawed) approach to minus does make the Algeblocks easier to use.
But for mathematical correctness, depth, and range, check out my Algebra Lab Gear: Basic Algebra and Algebra Lab Gear: Algebra 1 (Where to get them.) They are the result of many years of work with algebra manipulatives, with both teachers and students, and include all the basic lessons on the distributive law, a rich introduction to equation solving, plus three important features:
For extended work on integer arithmetic, plus a general introduction to Lab Gear at a more basic level, see The Algebra Lab: Middle School. (Where to get it.) For many lessons using algebra manipulatives, including some seminal ones, and some available absolutely nowhere else (e.g. a model of polynomial long division), see The Algebra Lab: High School, a whole book in PDFs available for free on this site.
Many of the best ideas I have come up with in this domain have been incorporated (unfortunately without attribution) in CPM's Algebra Connections, a rather wonderful Algebra 1 textbook. This includes everything from the engaging perimeter problems, to using "make a rectangle" puzzles prior to the introduction of the distributive law, to "which is greater?" as a lead-in to equation solving, to the step-by-step introduction to completing the square. Hats off to the authors for their ability to recognize a good activity when they see its effectiveness with students, and many thanks to them for bringing these approaches to a much larger population than I could.
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The first use of manipulatives to illustrate algebraic ideas was by math educator Zoltan Dienes, who used base-10 blocks. Using the "rod" (10) as x, and the "flat" (100) as x^{2}, he showed how to use base 10 blocks to illustrate the distributive law. For example, (x+5)(x+2)=x^{2}+7x+10 can be seen by making a 12 by 15 rectangle, and seeing that its area is 100+7·10+10. The idea was powerful, and launched the idea of algebra manipulatives, but this model falls short when trying to factor: x^{2}+7x+10, represented by 100+7·10+10 can be arranged into two rectangles: 12 by 15, or 10 by 18. The first one is the correct factoring (x+2)(x+5), but the second is not: x(x+8) does not equal x^{2}+7x+10.
Mary Laycock improved on Dienes' model, by using multi-base blocks. Instead of just working with base 10 blocks, a trinomial factoring had to work in all bases. In other words, the same layout should work whether x=3, 5, 6, or 10. She also introduced the "upstairs" representation of minus, which made it possible to represent a product involving minus like (x-1)(x+1) in a geometrically correct way.
Peter Rasmussen used base ten tiles, plus 5-, 25-, and 50-tiles for convenience. More importantly, he created the non-commensurable x, which solved the problem of false factorings encountered when using arithmetic blocks for variables. He also laid out multiplications in a tray that was a precursor to the corner piece. His MathTiles were placed along the tray's frame in such a way that the student saw them edge-on, thereby suggesting the one-dimensionality of linear measurements of the dimensions of the two-dimensional rectangles inside the tray. His model of minus combined Laycock's upstairs method with a color scheme. The tiles were only painted on one side, so that if a tile is turned over to its unpainted side, it is considered negative. (Number tiles, x-tiles, and x^{2}-tiles each had their own color.)
The Algebra Tiles were based on Rasmussen's ground-breaking model, without the upstairs representation of minus. Unfortunately, they dropped the multiplication tray. Fortunately, they did keep the non-commensurable x.
The Lab Gear was based on Rasmussen's and Laycock's models, keeping the non-commensurable x, and adding a non-commensurable y, and blocks for 5x, 5y, and 25, for convenience. (Blocks for 10, 100, and 1000 can be readily added to the Lab Gear by using base 10 blocks, which could be quite useful in younger grades.) The corner piece extends the uses of the tray for multiplication into the third dimension, and 3D multiplication is supported by x^{3}, x^{2}y, y^{2}x, and y^{3} blocks. The minus model is extended beyond the upstairs model by the use of the workmat, with its minus area. The workmat is an environment for working with polynomials involving minus signs, as well as for equation solving and inequalities.
Algeblocks are based on the Lab Gear, including the x and y variables, and variations on the workmat. However, the 5, 25, 5x, 5y, are missing. The corner piece is replaced by the quadrant mat, and the upstairs representation of minus is abandoned.
Finally, the Algebra Models combine ingredients from all these sources: tiles rather than blocks, x and y variables, minus indicated by color, the Work Tray instead of the corner piece, a 5 block, but no 25, 5x, or 5y.
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