Strengthening Multiplicative Reasoning with Prime Numbers

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Strengthening Multiplicative Reasoning with Prime Numbers Matt Roscoe University of Montana [email protected]

Ziv Feldman Boston University [email protected]

NCTM Research Conference San Francisco, CA April 15, 2016

1

Rationale

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Setting the Stage

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Explore the Landscape of Prime Factorizations 1-100

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Constructing Factor Lists Using Primes

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What It’s All About: The FTA

6

Conclusion

Rationale

“A mathematician, like a painter or poet, is a maker of patterns. If his patterns are more permanent than theirs, it is because they are made with ideas.” — G. Hardy (from A Mathematician’s Apology, London 1941).

Rationale

“...in mathematics, the primary subject-matter is not the mathematical objects but rather the structures in which they are arranged” (Resnik, 1999, p.201).

Rationale

Common Core State Standards for Mathematics (2010) Operations and Algebraic Thinking (4.OA) 4. Find all factor pairs for a whole number in the range 1-100. Recognize that a whole number is a multiple of each of its factors. Determine whether a given whole number in the range 1-100 is a multiple of a given one-digit number. Determine whether a given whole number in the range 1-100 is prime or composite. (p. 29)

Rationale

Common Core State Standards for Mathematics (2010) Operations and Algebraic Thinking (4.OA) 4. Find all factor pairs for a whole number in the range 1-100. Recognize that a whole number is a multiple of each of its factors. Determine whether a given whole number in the range 1-100 is a multiple of a given one-digit number. Determine whether a given whole number in the range 1-100 is prime or composite. (p. 29)

Rationale

A richer understanding of factors and multiples can be developed by drawing attention to the important role that prime numbers play when two numbers are multiplicatively compared.

Setting the Stage

Any whole number greater than zero with exactly two distinct, positive divisors is a prime number, or simply prime. Examples: 2, 3, 5, 7, 11

Setting the Stage

Any whole number greater than zero that has a positive factor other than 1 and itself is a composite number, or simply composite. Examples: 4, 6, 8, 10, 12

Setting the Stage The Sieve of Eratosthenes 1

Create a list of consecutive whole numbers starting from 2 to any n

2

The first prime is 2, let p “ 2

3

Circle p. Starting from p count up in multiples of p and eliminate each of these numbers (i.e. 2p, 3p, 4p, ....) until you reach n

4

Next, find the first number greater than p in the list that is not marked; let p equal this number and repeat the previous step

5

Continue this process until the list is exhausted, the circled numbers are the prime numbers up to n

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Setting the Stage

Factor Tree Method

Setting the Stage

Factor Lattice Method 72

Setting the Stage

Factor Lattice Method

2

72 36

Setting the Stage

Factor Lattice Method

2 2

72 36 18

Setting the Stage

Factor Lattice Method

2 2 2

72 36 18 9

Setting the Stage

Factor Lattice Method

2 2 2 3

72 36 18 9 3

Setting the Stage

Factor Lattice Method

2 2 2 3

72 36 18 9 3

Explore the Landscape of Prime Factorizations 1-100

You each are given a collection of numbers. Do the following: 1

2

Construct each number as a product of primes and find a collection of blocks to represent the number and construct a “prime tower”. Once you have constructed all your towers, discuss the following questions: Which pairs of numbers share factors? Which pairs do not? How do you know? 2 How can you find the largest factor that a pair of numbers share? How do you know? 3 Are there any pairs where one number is a multiple of the other? How do you know? 1

Explore the Landscape of Prime Factorizations 1-100

For each of the numbers you have been given, place that number’s “prime tower” on the appropriate location on the grid. Once this landscape has been constructed, do the following: 1

THINK: Identify at least three patterns you notice.

2

PAIR: Discuss what you noticed with a partner.

3

SHARE: Share one pattern with the large group.

Constructing Factor Lists Using Primes

Your instructor will assign your group a number: 96

90

84

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78

72

Using the prime factorization grid, highlight the cells of all the factors of that number. What patterns do you notice?

Constructing Factor Lists Using Primes

Constructing Factor Lists Using Primes

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

Constructing Factor Lists Using Primes “Seeing” all the factors of 70.

What It’s All About: The FTA

The Fundamental Theorem of Arithmetic (FTA) states that every positive integer (except the number 1) can be represented in exactly one way apart from rearrangement as a product of one or more primes (Hardy and Wright 1979).

What It’s All About: The FTA

FTA Consequence 1: Subset/Set comparisons of prime factorizations align with factor/multiple relationships

What It’s All About: The FTA

FTA Consequence 2: The greatest common factor of two numbers can be determined by finding the largest collection of prime factors that the two numbers share.

What It’s All About: The FTA

FTA Consequence 3: The least common multiple of two numbers can be determined by finding the smallest collection of prime factors that include both numbers.

Conclusion

Students can develop a richer understanding of the multiplicative structure of number through exposure to the “fundamental” role that prime numbers play.

Thank You! Matt Roscoe [email protected]

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Prime Quantity Quantity Required To Produce 107 2 97 53 3 48 26 5 24 18 7 16 10 11 9 8 13 7 6 17 5 6 19 5 5 23 4 4 29 3 4 31 3 3 37 2 3 41 2 3 43 2 3 47 2 2 53 1 2 59 1 2 61 1 2 67 1 2 71 1 2 73 1 2 79 1 2 83 1 2 89 1 2 97 1 239 279

Group N1 N2 N3 N4 N5 N6 N7 N8 N9

Prime Prime Multiple Multiple Relatively Prime Extra Extra Extra Extra

1 97 61 30 60 77 42 56 81 93

2 31 67 32 64 45 44 57 82 94

3 37 71 33 66 40 46 58 84 95

4 41 73 34 68 63 48 62 86 96

5 43 79 35 70 75 49 65 87 98

6 47 83 36 72 55 50 69 88 99

7 53 89 38 76 51 52 74 90 100

8 59 91 39 78 85 54 80 92 28