The sieve for prime numbers
Write the numbers 1 to 50 and cross out multiples until only the primes are left, then find out where to stop.
📖 Start here: read the steps of the “Lesson” in order (1, 2, 3 …) and answer each question. Then continue with “Practice”.
- Iran: grade 8
Guide
This studio follows the Iranian Grade 8 lesson on finding primes. A prime number has exactly two factors, 1 and itself. 1 is not prime because it has only one factor.
The sieve: write 1 to 50. Put 1 aside. Cross out the multiples of 2 (but not 2 itself), then of 3, then of 5 and then of 7. The numbers not crossed out are prime.
To test one number you do not need every smaller number. It is enough to try the primes whose square is not bigger than the number.
If no such prime divides the number, it is prime number; otherwise it is composite number.