I need a song to help remember the prime numbers less than 100.

[Hot Lava Mama]

Anyone know of any really good ones? Preferrably free, but if it's good, it doesn't have to be free.

Thanks!

Hot Lava Mama

[K&Rs Mom]

:bigear: Anybody? Song, poem, chant, other memory trick?

[regentrude]

Don't you learn the 'characteristics of divisibility' for that?

IMO it is more helpfull to learn that then memorizing prime-numbers.

 

:iagree:

Memorizing prime numbers is not useful for anything - determining whether a given number is prime is.

[PeterPan]

You mean they aren't just interesting enough that you remember them? I used to wake up to them each day (wake at 7:17, that type thing). But I agree with the others, the divisibility rules are more important. You take a 100 chart and go through each of the patterns (2's, 3's, etc.) and then circle the primes that are left. Then these become your fun numbers to think about. Take 31, it's such an interesting number, not quite to 32 with all its factoring options, not a nice 30 that would be 1/2 an hour, just an oddball... Just make friends with them I guess. Or learn the factoring rules. :)

[wapiti]

:iagree: I don't think there's a point to memorizing the prime numbers.

 

By the way, I found a fun website about the Sieve of Erastothanes that some of you might enjoy:

http://www.1729.com/math/integers/sieve.html

 

though I think it's more useful for a student to go through a 100 chart themselves to mark off multiples and circle the primes.

[Dana]

:iagree: I don't think there's a point to memorizing the prime numbers.

 

By the way, I found a fun website about the Sieve of Erastothanes that some of you might enjoy:

http://www.1729.com/math/integers/sieve.html

 

though I think it's more useful for a student to go through a 100 chart themselves to mark off multiples and circle the primes.

 

There's also an illustration of it on Wikipedia.

[Hunter]

How can a number be prime if it's in the 2, 3 or higher column? Primes are only in the 1s column, right?

 

I too want to memorize the primes. For now I printed out a "Prime number chart", as well as a "20 x 20 multiplication chart" and a "squares, cubes and Square root chart" for 1-20 and penciled in x pi in the margin.

 

Having these 3 charts has helped me tremendously in getting my Saxon done quicker and more accurately. Now I need a plan to memorize at least some of this.

[Dana]

How can a number be prime if it's in the 2, 3 or higher column? Primes are only in the 1s column, right?

 

 

Nope.

 

Prime numbers are any natural number with exactly two factors.

 

One is neither prime nor composite (only has one factor).

Two is the only even prime.

 

The Sieve of Eratosthenes is a method to sort through numbers and list primes relatively rapidly. There are 25 primes less than 100.

(1) List the numbers 1-100.

(2) One isn't prime or composite, so ignore it.

(3) Circle two. Two is a prime number (only factors are 1 and 2).

(4) Go through your chart and cross off every number that is a multiple of two. (You've now crossed off every even number other than two.)

(5) The next number is three. Circle it. It's prime (only factors are 1 and 3).

(6) Go through your chart and cross out every multiple of three. Note that some of these have already been crossed out (like 6).

(7) Continue this procedure with the next numbers... next will be with 5 (since 4 was crossed out) and you'd cross out all multiples of 5.

 

You can check your work against the wiki entry for the sieve... it lists the primes under 130 (I think).

 

In college, I ran across a book in the library from the early 1900s. It listed the first 20,000 primes (or the primes less than 20,000). Wild to have to look it up... but primes are very useful!

[Hunter]

I see what I was doing wrong. I was confusing a 100 chart with a 10 X 10 multiplication chart. There are no primes on a 10 x 10 chart other than in the 1s column, right?

[kiana]

I see what I was doing wrong. I was confusing a 100 chart with a 10 X 10 multiplication chart. There are no primes on a 10 x 10 chart other than in the 1s column, right?

 

(and the 1s row, to be pedantic)

 

But yes. Anything outside the 1s column/row the product of the two numbers which define its place in the table, and since neither of those two numbers are 1 it must be composite.

[PeterPan]

We've done that Eros. sieve exercise several times in various math curricula (BJU, RS, etc.). It's a lot of fun! :)

[Hunter]

The OP wants to memorize the prime numbers and so do I. I'm tired of having to figure them out or look them up. Can anyone humor us and help us out, even if you think it's a useless waste of time :-)

[Guest cashclan]

My FAVORITE Prime Number Song can be found here 

http://www.songsforteaching.com/recall/primenumbers.htm

 

It's on the Musical Recall's Greatest Hits Vol 1, or you can just purchase the individual download.

 

Here are the words, it has a catchy tune. You may listen to a sample at the link...

 

Prime numbers are only divisible
By themselves and the number "one."
Prime numbers are only divisible
By themselves and the number "one."

2 3 5 7 11 13 17 19 23

29 31 37 41 43 47 53 59 61 67

71 73 79 83 89 97 101

 

[boscopup]

Another thing to remember is that you only have to check up through "10" for numbers up to 100, because 10^2=100, so if the number is a multiple of a number greater than 10, it will also have to have a factor less than 10. So you only need to check up through 10, and the only prime numbers through 10 are 2, 3, 5, and 7.

 

Even numbers are multiples of 2.

Digit sum that is divisible by 3 will be a multiple of 3.

Numbers that end in 0 or 5 will be divisible by 5.

And for 7, I'd just do some quick short division in my head. :tongue_smilie:

 

So if you need to know if a number less than 100 is prime, just ask yourself if it's divisible by any of the above (2, 3, 5, 7), and if it's not, then it is prime.

 

Oh, and my oldest son LOVES the book You Can Count on Monsters. It talks about factoring and prime numbers. Great little book.

[8filltheheart]

The OP wants to memorize the prime numbers and so do I. I'm tired of having to figure them out or look them up. Can anyone humor us and help us out, even if you think it's a useless waste of time :-)

Do you know the rules for divisibility? They definitely help with revealing composite numbers, even beyond 100. Even and 5s and 0s are obvious. But perhaps you didn't know that if a number's digits add up to a number divisible by 3, it is divisible by 3. Simple example is 21. 2+1=3, so 21is divisible by 3.

 

Divisibility by 11: http://www.math-help-ace.com/Divisibility-By-11.html

To find out, if a number is divisible by 11, take the sum of the digits at odd places, and the sum of the digits at even places, and subtract one from the other. If you get an answer divisible by 11 (including zero), then the original number is divisible by 11.

If you don't know the new number's divisibility, you can apply the rule again.(usually, it is not required to apply again.)

 

Example 1 of Divisibility Rule for 11

 

Find whether 65714 is divisible by 11 or not.

 

Solution :

sum of alternate digits = 6 + 7 + 4 = 17; sum of remaining digits = 5 + 1 = 6;

Subtracting, 17 - 6 = 11 is divisible by 11.

∴ 65714 is divisible by 11. Ans.

 

Example 2 of Rule of Divisibility By 11

 

Find whether 123453 is divisible by 11 or not.

 

Solution :

sum of alternate digits = 1 + 3 + 5 = 9; sum of remaining digits = 2 + 4 + 3 = 9;

Subtracting, 9 - 9 = 0.

∴ 123453 is divisible by 11. Ans.

 

 

There are multiple ways to find divisibility by 7. 13 has ways, etc. If you google divisibility rules, they are easy to find and pretty useful with numbers outside the multiplication tables memorized.

http://bobprior.com/ForTeachers/DivisibilityRule7.pdf

[Hunter]

This is an OLD thread :lol:

 

Still a good one, though. Thanks everyone.

[boscopup]

This is an OLD thread :lol:

 

Still a good one, though. Thanks everyone.

Oops! I didn't know the divisibility by 11 rule though, so a good thread anyway! :D