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# Finding factors of a number Use the beans to find factors of 24  Count out 24 beans  We know that products can be illustrated using a rectangular model.

## Presentation on theme: "Finding factors of a number Use the beans to find factors of 24  Count out 24 beans  We know that products can be illustrated using a rectangular model."— Presentation transcript:

Finding factors of a number Use the beans to find factors of 24  Count out 24 beans  We know that products can be illustrated using a rectangular model  Make a rectangle using the beans  What are the numbers you multiply to get 24?  Can you arrange the beans into a different rectangle?  What product does this represent?

 How many different rectangles can you make?  Count out 11 beans.  How many rectangles can you make with 11 beans?

Sieve of Eratosthenes Eratosthenes was born in Cyrene which is now in Libya in North Africa in 276 BC. He died in 194 BC. Eratosthenes made a surprisingly accurate measurement of the circumference of the Earth. He was also fascinated with number theory, and he developed the idea of a sieve to illustrate prime numbers.

The Sieve of Eratosthenes Prime Number Divisible only by 1 and itself Finding prime numbers using the sieve

Sieve of Eratosthenes You will need many different colors. Use one color for each factor. Circle the number “1”. 1 is neither prime nor composite, as we have seen earlier. Now, circle 2. Every multiple of 2 is a composite number, so put a dot of that color next to all of the multiples of 2. Use a new color. Now, circle 3. Every multiple of 3 is a composite number, so put a dot of this new color next to all multiples of 3.

Sieve of Eratosthenes Now, 4 has a dot next to it--it is not prime. Skip it and move on. Use a new color. Circle 5, and then put a dot of this new color next to all multiples of 5. Now, 6 has a dot next to it--it is not prime. Skip it and move on. Continue until you know that only prime numbers are left. When can you stop? How do you know?

Sieve of Eratosthenes Questions to answer: When you circled 11, were there any multiples of 11 that did not already have dots next to them? Can you explain to a child why this was true? What does this have to do with factors and multiples? What are the prime numbers that are between 1 and 100? Is 1 a prime number?

Sieve of Eratosthenes 12345678910 11121314151617181920 21222324252627282930 31323334353637383940 41424344454647484950 51525354555657585960 61626364656667686970 71727374757677787980 81828384858687888990 919293949596979899100

Sieve of Erathosthenes

Names for these numbers 11 is an example of a 24 is an example of a

Factors of 24 List How should they be ordered? How do you know you have them all?

Factors of 24--How do we know when we have them all? 1 12 2 24 3 8 4 6

Exploration 4.2 First, fill in the table on page 85, using the information on the sieve. It will help if you write them in pairs. For example, for 18: 1, 18; 2, 9; 3, 6. The order does not matter. Next, fill in the table on page 87. Use the table on page 85 to help.

Find all of the factors of 30

Now find all of the factors of 60

Factorization Factorization is writing a number as a product of factors. 24 60

Prime Factorization A factorization of the number in which all of the factors are prime numbers. 10 12

Prime Factorization 24 25

Prime Factorization Using a factor tree to do prime factorization. 60

112

Exploration 4.3 is due on Thursday #1,2,6,7,8 along with some exercises from the textbook. Please put the exploration on a separate paper than the textbook problems.

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