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?
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.