Prime Numbers With Mrs Ford

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  A sieve has holes in it and is used to filter out the juice.  Eratosthenes’s  Eratosthenes’s sieve filters out numbers to find the prime numbers.

Definition Definition  Factor Factor – a number that is multiplied by another to give a product. 7 x 8 = 56 Factors

Definition  Factor Factor – a number that divides evenly into another. 56 ÷ 8 = 7 Factor

Definition Prime Number Number – a number that has only two factors, itself and is prime because the only numbers that will divide into it evenly are 1 and 7.

Hundreds Chart  On  On graph paper, make a chart of the numbers from 1 to 100, with 10 numbers in each row.

Hundreds Chart

– Cross out 1; it is not prime.

Hint For Next Step Remember all numbers divisible by 2 are even numbers.

– Leave 2; cross out multiples of 2

Hint For Next Step  To  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 Total of digits = 15 3 divides evenly into is a multiple of 3

– Leave 3; cross out multiples of 3

 To  To find the multiples of 5 look for numbers that end with the digit 0 and 5. Hint For the Next Step 385 is a multiple of 5 & 890 is a multiple of 5 because the last digit ends with 0 or 5.

– Leave 5; cross out multiples of 5

– Leave 7; cross out multiples of 7

–Leave 11; cross out multiples of 11

All the numbers left are prime

The Prime Numbers from 1 to 100 are as follows: 2, 3, 5, 7, 11, 13, 17, 19, 23, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97

THE END