Using the Area Model to Teach Multiplying, Factoring and Division of Polynomials

visit us at www.cpm.org Using the Area Model to Teach Multiplying, Factoring and Division of Polynomials For more information about the materials presented, contact Chris Mikles mikles@cpm.org From CCA and CCA2

-5. DIAMOND PROBLEMS Finding and using a pattern is an important problem-solving skill you will use in algebra. The patterns in Diamond Problems will be used later in the course to solve other types of algebraic problems. Look for a pattern in the first three diamonds below. For the fourth diamond, eplain how you could find the missing numbers (?) if you know the two numbers (#). 0 5 2 7 6 2 5 4 4 5? # #? Etra Copy the Diamond Problems below onto your paper. Then use the pattern you discovered to complete each one. a. b. c. d. e. 2 4 7 6 2 5 2 4 7 4 8 8 2 2 8 2 4 6-25. Your teacher will distribute a set of algebra tiles for your team to use during this course. a. The tiles have a positive side and a negative side. In this tet the positive side will be the shaded side. Flip the tiles so that the positive side of each tile is facing up. Trace one of each of the si tiles provided by your teacher on your paper. Leave plenty of space between each tracing. b. The dimensions of some of the tiles are shown at right. Label the dimensions of all the tiles net to the tracings you made. y y c. The algebra tiles will be named according to each of their areas. Write the name of each tile in the center of your tracing with a colored pen or pencil. Make the name of the tile stand out. [,, y, y, 2, and y 2. ] d. Below each tile write P = and then find the perimeter of each tile. [ -tile = 4, -tile = 2 + 2, y-tile = 2y + 2, y-tile = 2 + 2y, 2 -tile = 4, and y 2 -tile = 4y. ] Etra Take 6 unit tiles and make a rectangle. Is there another rectangle that you can make? Draw the rectangles with the dimensions labeled. Write down all the dimensions of the rectangles whose area equals 6 square units. These are the factors of 6. CPM Educational Program 20, all rights reserved 2

-45. Your teacher will put this group of tiles on the overhead: [ There is one 2- tile, si -tiles, and eight unit tiles. ] 2 a. Using your own tiles, arrange the same group of tiles into one large rectangle, with the 2 tile in the lower left corner. On your paper, sketch what your rectangle looks like. [ See the Suggested Lesson Activity for a possible rectangle. ] b. What are the dimensions (length and width) of the rectangle you made? Label your sketch with its dimensions, then write the area of the rectangle as a product, that is, length width. [ ( + 2)( + 4) ] c. The area of a rectangle can also be written as the sum of the areas of all its parts. Write the area of the rectangle as the sum of its parts. Simplify your epression for the sum of the rectangle s parts. [ 2 + 6 + 8 ] d. Write an equation that shows that the area written as a product is equivalent to the area written as a sum. [ 2 + 6 + 8 = ( + 2)( + 4) ] -46. Your teacher will assign several of the epressions below. For each epression, build a rectangle using all of the tiles, if possible. Sketch each rectangle, find its dimensions, and write an epression showing the equivalence of the area as a sum (like 2 + 5 + 6) and as a product (like ( + )( + 2)). If it is not possible to build a rectangle, eplain why not. a. 2 + + 2 b. 6 + 5 c. 2 2 + 7 + 6 d. 2 + 4 + e. 2 2 +0 +2 CPM Educational Program 20, all rights reserved

ETHODS AND MEANINGS Multiplying Algebraic Epressions with Tiles MATH NOTES The area of a rectangle can be written two different ways. It can be written as a product of its base and height or as a sum of its parts. For eample, the area of the shaded rectangle at right can be written two ways: area as a product (!"# + 4) (!"# + 2)!!!=!!!2! $ + " 6 $ + # 8 base height area as a sum area 2-48. For the entire rectangle at right, find the area of each part and then find the area of the whole. [ 77 + 56 + + 24 = 90 square units ] 7 8-49. Write the area of the rectangle at right as a product and as a sum. [ (2 + 4)( + 2) = 2 2 + 8 + 8 ] 2 2-54. For each of the following rectangles, find the dimensions (length and width) and write the area as the product of the dimensions and as the sum of the tiles. Remember to combine like terms whenever possible. a. b. 2 y y [ (2 + )( + 2) = 2 2 + 7 + 6 ] [ (y + + 2)( + 4) = y + 2 + 6 + 4y + 8 ] -55. Your teacher will assign your team some of the epressions below. Use your algebra tiles to build rectangles with the given dimensions. Sketch each rectangle on your paper, label its dimensions, and write an equivalence statement for its area as a product and as a sum. Be prepared to share your solutions with the class. a. ( + )(2 +) [ 2 2 + 7 + ] b. 2( + 5) [ 2 2 + 0 ] e. (2 +)(2 +) [ 4 2 + 4 + ] f. (2)(4) [ 8 2 ] CPM Educational Program 20, all rights reserved 4

g. 2( + 5) [ 6 + 0 ] -56. With your team, eamine the solutions you found for parts (b) and (g) of problem -55. This pattern is called the Distributive Property. Multiply the following epressions without using your tiles and simplify. Be ready to share your process with the class. a. 2(6 + 5) [ 2 2 + 0 ] b. 6(4 +) [ 24 + 6 ] c. y(4 + ) [ 2y + 9y ] d. 7y(0 +y) [ 70y + 77y 2 ] -58. Eamine the rectangles formed with tiles below. For each figure, write its area as a product of the width and length and as a sum of its parts. a. b. [ ( + )( + ) = 2 + 4 + ] [ (2 + )( + 2) = 2 2 + 5 + 2 ] -59. Find the total area of each rectangle below. Each number inside the rectangle represents the area of that smaller rectangle, while each number along the side represents the length of that portion of the side. a. [ 28 square units ] b. [ 2 square units ] 2 9 4 40 8 8 6-65. Write the area as a product and as a sum for the rectangle shown at right. [ (4 + 5)(2 + ) = 8 2 + 22 + 5 ] 2 CPM Educational Program 20, all rights reserved 5

-66. Now eamine the following diagram. How is it similar to the set of tiles in problem -65? How is it different? Talk with your teammates and write down all of your observations. [ Students should notice that the area inside each smaller rectangle of the generic rectangle corresponds to the tiles in the same portions of the rectangle in problem -65, but it does not show the individual tiles. ] 2 5 2 8 2 0 4 5-67. Diagrams like the one in problem -66 are referred to as generic rectangles. Generic rectangles allow you to use an area model to multiply epressions without using the algebra tiles. Using this model, you can multiply with values that are difficult to represent with tiles. Draw each of the following generic rectangles on your paper. Then find the area of each part and write the area of the whole rectangle as a product and as a sum. a. [ ()(y + 5) = y + 5 ] b. [ ()(2) = 2 2 ] y +5 2 c. [ ( + 5)(2! ) = 2 2 + 7! 5 ] d. [ (4y! 7)(6y! ) = 24y 2! 46y + 7 ] +5 7 4y 6y 2 e. How did you find the area of the individual parts of each generic rectangle? [ Answers vary. ] -68. Multiply and simplify the following epressions using either a generic rectangle or the Distributive Property. For part (a), verify that your solution is correct by building a rectangle with algebra tiles. a. ( + 5)( + 2) b. (2y! 5)(5y + 7) [ 2 + 7 + 0 ] [ 0y 2! y! 5 ] CPM Educational Program 20, all rights reserved 6

c. (6 2!y) d. (5w! 2 p)(w + p! 4) [ 8! y ] [ 5w 2! wp! 2p 2! 20w + 8p ] -69. THE GENERIC RECTANGLE CHALLENGE Copy each of the generic rectangles below and fill in the missing dimensions and areas. Then write the entire area as a product and as a sum. Be prepared to share your reasoning with the class. a. [ y( + ) = y + y ] b. [ ( + 2)( + 5) = 2 + 7 + 60 ] y y c. [ (! 2)(2! y + 5) = 2 2! y + + 6y! 0 y 2 4 0 2 2 5 ] d. [ Multiple answers possible. ] 2 6 y -8. Find the dimensions of the generic rectangle at right. Then write an equivalency statement (length width = area) of the area as a product and as a sum.(! 5)( + ) = 2! 2! 5 2 5 5 8-2. The process of changing a sum to a product is called factoring. Can every epression be factored? That is, does every sum have a product that can be represented with tiles? Investigate this question by building rectangles with algebra tiles for the following epressions. For each one, write the area as a sum and as a product. If you cannot build a rectangle, be prepared to convince the class that no rectangle eists (and thus the epression cannot be factored). [ a: (2 + )( + 2), b: (2 + )( + 2), c: no solution, d: (2 + y)(y + ) ; Conclusion: Not every epression can be factored. ] a. 2 2 + 7 + 6 b. 6 2 + 7 + 2 c. 2 + 4 + d. 2y + 6 + y 2 + y 8-. Work with your team to find the sum and the product for the following generic rectangles. Are there any special strategies you discovered that can help you determine the dimensions of the rectangle? Be sure to share these strategies with your teammates. [ a: ( +)(2 + 5) = 6 2 + 7 + 5, b: (5! 2)(y + ) = 5y +5! 2y! 6, c: (4! )( + 4) = 2 2 + 7! 2 ] CPM Educational Program 20, all rights reserved 7

a. b. c. 2 5!2y!6!9!2 6 2 5 5y 5 2 2 6 8-4. While working on problem 8-, Casey noticed a pattern with the diagonals of each generic rectangle. However, just before she shared her pattern with the rest of her team, she was called out of class! The drawing on her paper looked like the diagram below. Can you figure out what the two diagonals have in common? [ The product of each diagonal is equal: 6 2! 5 = 0 2 and 2!5 = 0 2. ] 2 5 6 2 5 8-4. FACTORING QUADRATICS To develop a method for factoring without algebra tiles, first study how to factor with algebra tiles, and then look for connections within a generic rectangle. a. Using algebra tiles, factor 2 2 + 5 + ; that is, use the tiles to build a rectangle, and then write its area as a product. [ (2 + )( +) ] b. To factor with tiles (like you did in part (a)), you need to determine how the tiles need to be arranged to form a rectangle. Using a generic rectangle to factor requires a different process. Miguel wants to use a generic rectangle to factor 2 + 0 + 8. He knows that 2 and 8 go into the rectangle in the locations shown at right. Finish the rectangle by deciding how to place the ten -terms. Then write the area as a product. [ One corner should contain 4, while the other should contain 6 ; ( + 4)( + 2). ] c. Kelly wants to find a shortcut to factor 2 2 + 7 + 6. She knows that 2 2 and 6 go into the rectangle in the locations shown at right. She also remembers Casey s pattern for diagonals. Without actually factoring yet, what do you know about the missing two parts of the generic rectangle? [ Their sum is 7, and their product is 2 2. ] 2? 2 2 8 6? CPM Educational Program 20, all rights reserved 8

d. To complete Kelly s generic rectangle, you need two -terms that have a sum of 7 and a product of 2 2. Create and product solve a Diamond Problem that represents this situation. [ The product 2 2 should be placed at the top of the diamond problem, 7 at the bottom, and terms and 4 should be in the middle. ] sum e. Use your results from the Diamond Problem to complete the generic rectangle for 2 2 + 7 + 6, and then write the area as a product of factors. [ (2 + )( + 2) ] 8-5. Factoring with a generic rectangle is especially convenient when algebra tiles are not available or when the number of necessary tiles becomes too large to manage. Using a Diamond Problem helps avoid guessing and checking, which can at times be challenging. Use the process from problem 8-4 to factor 6 2 + 7 + 2. The questions below will guide your process. a. When given a trinomial, such as 6 2 + 7 + 2, what two parts of a generic rectangle can you quickly complete? [ One corner contains 6 2, and the opposite corner contains 2. ] b. How can you set up a Diamond Problem to help factor a trinomial such as 6 2 + 7 + 2? What goes on the top? What goes on the bottom? [ The product of the 2 and units terms (in this case, 72 2 ) goes on top, while the -term (7 ) goes on bottom. See diagram at right. ] product 72 2 9 8 7 sum c. Solve the Diamond Problem for 6 2 + 7 + 2 and complete its generic rectangle. [ See diagrams at right. ] +4 2 8 + 2 d. Write the area of the rectangle as a product. [ (2 + )( + 4) 6 2 9 8-6. Use the process you developed in problem 8- to factor the following quadratics, if possible. If a quadratic cannot be factored, justify your conclusion. [ a: ( + )( + 6), b: (4! )( + 5), c: (2! )(2! ), d: not factorable because there are no integers that multiply to get!9 2 (the diagonal of the generic rectangle) and add to get 5. ] a. 2 + 9 +8 b. 4 2 +7!5 c. 4 2!8 + d. 2 + 5! 8-00. COMPLETING THE SQUARE Jessica was at home struggling with her Algebra homework. She had missed class and did not understand the new method called completing the square. She was supposed to use it to change y = 2 + 8 +0 to graphing form. Then her precocious CPM Educational Program 20, all rights reserved 9

younger sister, who was playing with algebra tiles, said, Hey, I bet I know what they mean. Anita s Algebra class had been using tiles to multiply and factor binomials. Anita eplained: 2 + 8 +0 would look like this; 2 Yes, said Jessica, I m taking Algebra too, remember? Anita continued, And you need to make it into a square! OK, said Jessica, and she arranged her tiles on an equation mat as shown at right. Oh, said Jessica. I need 6 small unit tiles to fill in the corner! + + 4 + y + 4 2 But you only have 0, Anita reminded her. Right, I only have ten, Jessica replied. She drew the _ outline of the whole square and said: Oh, I get it! To complete the square, I need to add si tiles to each side of the equation: _ + + y + 4 + 4 2 CPM Educational Program 20, all rights reserved 0

Oh, I see, said Anita. You started with y = 2 + 8 +0, but now you can rewrite it as y + 6 = ( + 4) 2. Thank you so much, Anita! Now I can easily write the function in graphing form, y = ( + 4) 2! 6. How can you use your graphing calculator to verify that y = 2 + 8 +0 and y = ( + 4) 2! 6 are equivalent functions? [ Enter them both in the calculator and verify the tables are the same. ] 8-02. Help Jessica with a new problem. She needs to complete the square to write y = 2 + 4 + 9 in graphing form. a. Draw tiles to help her figure out how to make this epression into a square. Does she have too few or too many unit squares this time? Write her function in graphing form. [ y = ( + 2) 2 + 5 ] b. Find the verte and the -intercepts. What happened? What does that mean? [ verte: (!2,!5). We had to take the square root of a negative; no real solution and no -intercepts. ] y c. Algebraically find the y-intercept. Sketch the graph. [ y-intercept at (0, 9); see graph at right. ] 8-0. How could you complete the square to change f () = 2 + 5 + 2 into graphing form? How would you split the five -tiles into two equal parts? Jessica decided to use force! She cut one tile in half, as shown below. Then she added her two small unit tiles. + 2.5 + 2.5 2 2 Figure A 2 + 2.5 + 2.5 2 2 Figure B 2 a. How many small unit tiles are missing from Jessica s square? [ 4.25 ] b. Write the graphing form of the function, name the verte and y-intercept, and sketch the graph. [ f verte: (!2.5,!!4.25) ; see graph at right. ] ( ) = ( + 2.5) 2! 4.25 ; y CPM Educational Program 20, all rights reserved

8-. Andre needs to find the eact roots of the function f () = + 2 2! 7! 2. When he uses his graphing calculator, he can see that one of the -intercepts is 2, but there are two other intercepts that he cannot see eactly. What does he need to be able to do to find the other roots? [ This is a discussion question. Students may or may not suggest that dividing by! 2 will yield a quadratic factor that they can set equal to zero and solve. ] Andre remembers that he learned how to multiply binomials and other polynomials using generic rectangles. He figures that since division is the inverse (or undo) operation for multiplication, he should be able to reverse the multiplication process to divide. As he thinks about that idea, he comes across the following news article. Polydoku Craze Sweeping Nation! (CPM) Math enthusiasts around the nation have entered a new puzzle craze involving the multiplication of polynomials. The goal of the game, which enthusiasts have named Polydoku, is to fill in squares so that the multiplication of two polynomials will be completed. 2 4 5 A! 2 2 + B 6 4! 9 2! C 2!4 2 2!6 2 6 4!7 + 2!9 +2 The game shown at right, for eample, represents the multiplication of ( 2)(2 2 + ) = 6 4 7 + 2 9 + 2. Most of the squares are blank at the start of the game. While the beginner level provides the factors (in the gray squares), some of the factors are missing in the more advanced levels. CPM Educational Program 20, all rights reserved 2

8-4. Andre decided to join the craze and try some Polydoku puzzles, but he is not sure how to fill in some of the squares. Help him by answering parts (a) and (b) below about the Polydoku puzzle in the news article he read (found in problem 9-4), then complete part (c). [ a:!2(! 2 ) = 2 2, b:! +!4, c: See completed puzzle below. ] a. Eplain how the term 2 2 in cell C of the news article was generated. b. What values were combined to get!7 in the news article answer? c. Copy and complete the Polydoku puzzle at right. 2 4 5 A! 4 +!6 2 2 5 B C 2 2 4 5 A! 4 +!6 2 2 5 B 2 8 4 2!4 2!0 C!2!8 2 6 5 8 4 +0!22 2!4 +5 8-5. POLYDOKU TEAM CHALLENGE Work with your team to complete the puzzle at right. Find the factors and the product for the puzzle. If you get stuck, you can consult parts (a) through (c) below for ideas. [ See completed puzzle below. (! 4)(2! 2! 2 + 4) = 2 4! + 0 2 + 2! 6, a: B2 is the same as the first term in the answer, b: B4 + C,!2 2 + 2 2, c: Answer term #4 = 2 and C4 = 8, B5 + (8) = 2, B5 = 4 ] 2 4 5 A!! 2 B 2 4 C! 4 2 2 2 2 4 5 A! 2! 2! 2 +4 B 2 4!!2 2 4 C! 4!8 2 2 8!6 2 4! +0 2 +2!6 a. How is cell B2 related to the answer? b. How did you find the third term in the answer? c. What cells did you use to get the value in cell B5? CPM Educational Program 20, all rights reserved

8-6. Jessica is about to start the intermediatelevel Polydoku puzzle shown at right. Show Jessica how to complete the puzzle. Make sure you can justify your solution. [ See completed puzzle below. ] A B 2 4! 2 Use your results to complete the statements below. [ 6 + 7 2!6+0 2+ 5 = 2! 4 + 2 and C 6 +!5 +7 2!6 +0 (2 + 5)( 2! 4 + 2) = 6 + 7 2! 6 + 0 ] 6 + 7 2! 6 + 0 2 + 5 = and (2 + 5)! = 2 4 A! 2!4 +2 B 2 6!8 2 4 C +!5 5 2!20 0 6 +7 2!6 +0 8-7. Unfortunately, Jessica made a mistake when she copied the problem. The constant term of the original polynomial was supposed to have the value + 8 (not + 0). She does not want to start all over again to solve the puzzle. [ a: It is the remainder. b: 2! 4 + 2 + 2 8 + 5, c: 2 2 +! 5 + 4 + ] a. Jessica realizes that she now has 8 remaining from the original epression. What is the significance of this 8? b. Jessica writes her work as shown below: 6 + 7 2! 6 + 8 2 + 5 = (6 + 7 2! 6 + 0) + 8 2 + 5 = 2! 4 + 2, remainder 8. Gina thinks that there is a way to write the answer without using the word remainder. Discuss this with your team and find another way to write the result. Be prepared to share your results and your reasoning with the class. c. Use Jessica and Gina s method to divide (6 + 2! 2! ) ( + ). CPM Educational Program 20, all rights reserved 4

8-8. Create your own Polydoku puzzles that can be used to solve each of the polynomialdivision problems below. Epress any remainders as fractions and use your results to write a multiplication and a division statement such as those in problem 9-7. [ a: 2! 2 +! 5, b:! 4 2! 8 + 2, c: 2 + 4! 2!!, d: 4 + + 2 + + ] a. 6 4! 5 + 0 2! 8 + 5! b. ( 4! 6 + 8! 4) (! 2) c.! + 2! 4 + d. 5!! 8-9. Now work with your team to help Andre solve his original problem (problem 9-4). Find all of the roots (eact zeros) of the polynomial. [ 2,!2 +,!2! ] ETHODS AND MEANINGS MATH NOTES Using long division:! 4 2! 8 + 2! 2 4! 6 + 0 2 + 8! 4! 2! 4 + 0 2! 4 + 8 2! 8 2 + 8! 8 2 + 6 2! 2! 4 2 Answer:! 4! 8 + 2 +! 2 Using generic rectangles: 4 4 Polynomial Division The eamples below show two methods for dividing 4! 6 + 8! by! 2. In both cases, the remainder is written as a fraction. 2! 4 8 + 2! 4-8 2 + 2 2! 2 + 8 2 + 6-4 2! 6 + 0 + 8 2 Answer:! 4! 8 + 2 +! 2! Remainder Therefore, ( 4! 6 + 8! ) (! 2) =! 4 2! 8 + 2 +!2 and (! 2)(! 4 2! 8 + 2 + +2 ) = 4! 6 + 8! CPM Educational Program 20, all rights reserved 5

Lesson.2.A Resource Page Page 2 of Algebra Tiles CPM Educational Program 20, all rights reserved 6

Lesson.2.A Resource Page Page of Algebra Tiles CPM Educational Program 20, all rights reserved 7

Mathematics Standards for Mathematical Practice The Standards for Mathematical Practice describe varieties of epertise that mathematics educators at all levels should seek to develop in their students. These practices rest on important processes and proficiencies with longstanding importance in mathematics education. The first of these are the NCTM process standards of problem solving, reasoning and proof, communication, representation, and connections. The second are the strands of mathematical proficiency specified in the National Research Council s report Adding It Up: adaptive reasoning, strategic competence, conceptual understanding (comprehension of mathematical concepts, operations and relations), procedural fluency (skill in carrying out procedures fleibly, accurately, efficiently and appropriately), and productive disposition (habitual inclination to see mathematics as sensible, useful, and worthwhile, coupled with a belief in diligence and one s own efficacy). Make sense of problems and persevere in solving them. Find meaning in problems Look for entry points Analyze, conjecture and plan solution pathways Monitor and adjust Verify answers Ask themselves the question: Does this make sense? 2 Reason abstractly and quantitatively. Make sense of quantities and their relationships in problems Learn to contetualize and decontetualize Create coherent representations of problems Construct viable arguments and critique the reasoning of others. Understand and use information to construct arguments Make and eplore the truth of conjectures Recognize and use countereamples Justify conclusions and respond to arguments of others 4 Model with mathematics. Apply mathematics to problems in everyday life Make assumptions and approimations to simplify a complicated situation Identify quantities in a practical situation Interpret results in the contet of the situation and reflect on whether the results make sense 5 Use appropriate tools strategically. Consider the available tools when solving problems Are familiar with tools appropriate for their grade or course ( pencil and paper, concrete models, ruler, protractor, calculator, spreadsheet, computer programs, digital content located on a website, and other technological tools) Make sound decisions of which of these tools might be helpful 6 Attend to precision. Communicate precisely to others Use clear definitions, state the meaning of symbols and are careful about specifying units of measure and labeling aes Calculate accurately and efficiently 7 Look for and make use of structure. Discern patterns and structures Can step back for an overview and shift perspective See complicated things as single objects or as being composed of several objects 8 Look for and epress regularity in repeated reasoning. Notice if calculations are repeated and look both for general methods and shortcuts In solving problems, maintain oversight of the process while attending to detail Evaluate the reasonableness of their immediate results CPM Educational Program 20, all rights reserved 8