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# Prime Numbers

## on Sep 03, 2009

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## Prime NumbersPresentation Transcript

• Prime NumbersEratosthenes’ Sieve
By Monica Yuskaitis
Eratosthenes(ehr-uh-TAHS-thuh-neez)
Eratosthenes was the librarian at
Alexandria, Egypt in 200 B.C.
Note every book was a scroll.
Eratosthenes(ehr-uh-TAHS-thuh-neez)
Eratosthenes was a Greek mathematician, astronomer, and geographer.
He invented a method for finding prime numbers that is still used today.
This method is called Eratosthenes’ Sieve.
Eratosthenes’ Sieve
A sieve has holes in it and is used to filter out the juice.
Eratosthenes’s sieve filters out numbers to find the prime numbers.
Definition
Factor – a number that is multiplied by another to give a product.
7 x 8 = 56
Factors
Definition
Factor – a number that divides evenly into another.
56 ÷ 8 = 7
Factor
Definition
Prime Number – a number that has only two factors, itself and 1.
7
7 is prime because the only numbers
that will divide into it evenly are 1 and 7.
Hundreds Chart
On graph paper, make a chart of the numbers from 1 to 100, with 10 numbers in each row.
Hundreds Chart
1 – Cross out 1; it is not prime.
Hint For Next Step
Remember all numbers divisible by 2 are even numbers.
2 – Leave 2; cross out multiples of 2
Hint For Next Step
To find multiples of 3, add the digits of a number; see if you can divide this number evenly by 3; then the number is a multiple of 3.
2 6 7
Total of digits = 15
3 divides evenly into 15
267 is a multiple of 3
3– Leave 3; cross out multiples of 3
Hint For the Next Step
To find the multiples of 5 look for numbers that end with the digit 0 and 5.
385 is a multiple of 5
& 890 is a multiple of 5
because the last digit
ends with 0 or 5.
4– Leave 5; cross out multiples of 5
5– Leave 7; cross out multiples of 7
6–Leave 11; cross out multiples of 11