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Teaching prime and composite numbers

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Teaching prime and composite numbers
Old 08-20-2008, 06:23 PM
 
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New to 5th here and was never good at this in school...remember which is which that is.

So, I am wondering if you have a neat and/or simple way of teaching your students about prime and composite numbers along with things such as teaching them to use factor trees and such.

TIA!


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Old 08-20-2008, 09:02 PM
 
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For primes and composites, I give the kids a hundreds grid and they have to get out two different colored pencils. We start with twos, color 2 one color (the color for primes), then color every other multiple of two the other color (color for composites), then we go on to 3, then on 4, we talk about the fact that it's composite so we skip it. Then move to 5, 7, 11, 13, etc, all the way to 100. Then they have a grid to look at. Soon, they get good at it. I always challenge them to be able to ramble off their primes faster than I can (I was forced to memorize them in school). As for trees, I don't really have any new ways to doing it, just the old fashioned way I was taught in school, but the kids do well with it.
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Old 08-21-2008, 04:50 AM
 
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When we're finding factors of composite numbers, we make factor rainbows. So a factor rainbow for 16 would have these numbers listed:

1, 2, 4, 8, 16 and then an arched line connects 1 and 16, another connects 8 and 2 and an arch is placed over 4. So we start by listing one and 16 and then move inward unitl there are no more numbers in the middle that are factors.--I teach fourth grade, but I think fifth graders might still like making rainbows?
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Prime and Composite
Old 08-21-2008, 04:54 PM
 
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I do the 100s chart and list multiples like a previous poster and also do factor rainbows. One thing I tell them to get them to remember is PRIme is a PRIvate party, only two people can attend (1 and itself). COMPosite is a COMPany party, everyone is invited (lots of people).

Melinda
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Old 08-21-2008, 07:48 PM
 
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I was also going to suggest the Sieve of Eratosthenes but snowdaisy beat me to it. It such a terrific reference. I even had students who still had it in tucked away in their notebooks at the end of the school year when it is one of the first things we did.


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hello
Old 08-26-2008, 02:24 PM
 
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i dont get how to see if a number is prime or composite really fast as fast as i multiply.
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Old 08-28-2008, 03:18 PM
 
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I actually teach it through my daily Calendar math..but basically, we find the arrays and factors (you can find those together) of the number. If it has MORE than 2 factors and MORE than 2 arrays, it is composite. If it has only 2 factors and 2 arrays, it is prime. We draw them out (as part of the calendar) so the kids can easily see how many factors and arrays there are. We focus mainly on the first 31 numbers (as the calendar only goes up to 31!) but once they really internalize it, it is much easier for them to see the numbers higher than 51.
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