# 6th Grade Math, 11/7/17 “It’s Prime!”

After launching our first investigation earlier in the week, we were ready to conclude our review of prime and composite numbers with a closer look at large Prime Numbers.

On Friday, we replicated an ancient Greek technique called the Sieve of Eratosthenes

The Sieve of Eratosthenes, is a simple algorithm for finding all prime numbers up to any given limit. It does so by marking as composite (i.e., not prime) the multiples of each prime, starting with the multiples of 2. Once I eliminate the multiples of 2 for a given set of numbers, I then determine which are multiples of 3 and eliminate any that were not already eliminated as multiples of 2…then 5, 7, 11, 13….until I can be sure that a number has no other whole number factors than 1 and itself.

Once we learned and practiced the procedure for crossing out the multiples of the prime numbers using the numbers 1-100, students were asked “What is the Largest Prime Number You Can Find that is Greater than 100”?

Students were instructed to only use numbers that they could verify using their own calculations (no calculators) and create a poster reflecting this work and the thinking they did in the process. Some students in 601 even realized that 601 is prime!

###### Students in 601 doing a ‘Gallery Walk” to see the work of other students in their class.

As we continue into Investigation 2 this week, we will begin exploring factors and multiples more in-depth as we warm up and strengthen our number sense for more specific Algebra work coming up in a few months.