Divide the class into small groups and pass out 50 square tiles to each group.
Draw a chart on the board, with one side being the straight skinny and one
being a rectangular form. Have the students use the tiles to determine if a
number can form a straight skinny only or both a straight skinny and a rectangle
(squares included). Once you have gotten to the number 7 ask the students if
they are noticing any patterns.

What column will the number 8 go under? (Test their theory with the
tiles)

What column will the number 9 go under? (Test their theory with the
tiles)
-1 to 100 sheet
-Square tiles
Explain that Eratosthenes, a mathematician who lived from
276—196 BC, developed a method to find prime numbers.
His method is known as the Sieve of Eratosthenes.
Show the students this method to find the prime numbers
from 1 to 100. Pass out a 1 to 100 sheet to each student
and display one copy on the document camera.
Begin by circling 2 and crossing out all multiples of 2 on the
sheet. Have students do the same on their sheets. Move to
the next unmarked number (3). Circle 3 then cross out all multiples of 3. Have
students follow suit on their sheets.
Have students continue circling the next unmarked number and crossing out its
multiples until no unmarked numbers are left on their sheets. The remaining
(circled) numbers are all prime numbers.
?
What are all the prime numbers between 1 and 100? 2, 3, 5, 7, 11, 13,
17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
?
Can you find any patterns?
Mathematicians have been working for years to try and figure out a pattern in
the prime numbers (none have been found as yet).

Are all odd numbers prime? No

Is 5 an even or odd number? Is there a number with a 5 in the ones
place that’s prime? Yes, 5

Is 75 prime or composite? Composite

Is 435 prime or composite? Composite
© 2015 Zeno, All Rights Reserved
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‘
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© 2015 Zeno, All Rights Reserved