SIEVE OF ERATOSTHENES

Use the Sieve of Eratosthenes to find all primes up to 100.

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THE SIEVE OF ERATOSTHENES

This ancient method finds all primes up to any number. Write numbers 1 to N. Cross out 1. Circle 2, then cross out all multiples of 2. Circle 3, cross out multiples of 3. Continue with each uncrossed number. All circled numbers are prime!

Sieve of Eratosthenes to 30

1Cross out 1 (not prime)

2Keep 2, cross out all multiples of 2

3Keep 3, cross out all multiples of 3

4Keep 5, cross out 25

5Remaining: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29

= 10

LEADER TIP

Cross out multiples of 2, 3, 5, 7 on a hundred grid. The numbers left standing are prime. It's like a number detective game!

BEFORE THIS LESSON, TRY

What Are Primes?

PRACTICE WITH THESE GAMES

Prime Hunt

Number Balloons

Skip Counting

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SIEVE OF ERATOSTHENES

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SIEVE OF ERATOSTHENES