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The beauty and symmetry of mathematics. I was never taught this. were you? (Original Post) IcyPeas Oct 2022 OP
The teachers did not teach us that trick, but we learned it from a joke. Chainfire Oct 2022 #1
Got this one from the nuns! calimary Oct 2022 #22
9 is special like that Marthe48 Oct 2022 #2
I remember that trick AZSkiffyGeek Oct 2022 #8
The "nines" were always easiest for me. calimary Oct 2022 #21
Three works, too, sort of! Staph Oct 2022 #38
3 is another number that is special Marthe48 Oct 2022 #41
That is a WOW! I just sent it to my DIL..Math Teacher.. asiliveandbreathe Oct 2022 #3
Yes LetMyPeopleVote Oct 2022 #4
All of the number sense tricks come in handy, TexasTowelie Oct 2022 #10
I also used to compete in slide rule which is now obsolete LetMyPeopleVote Oct 2022 #13
I cannot believe how many people I taught this little reconciliation trick to: niyad Oct 2022 #15
I remember that from a basic Acctng class! liberalla Oct 2022 #18
That trick we used in the bank for years when looking for an outage. Bev54 Oct 2022 #19
We were taught this plus casting out nines unc70 Oct 2022 #5
I taught it for years in elementary teach1st Oct 2022 #6
My 1st graders couldn't figure it out but BigmanPigman Oct 2022 #20
I wish that I had been taught math like that. wnylib Oct 2022 #29
"I was taught by rote. Fortunately, I have a good memory" Me too but I have a crap memory. mitch96 Oct 2022 #35
LOL. My father was so good at mental calculations wnylib Oct 2022 #37
"languages and literature." I'll take Language and Lit any day of the week over "the maths" mitch96 Oct 2022 #40
I have studied Latin, German, and Spanish. wnylib Oct 2022 #42
I can't remember if I was taught it or found out on my own EYESORE 9001 Oct 2022 #7
A bit off topic but.. skypilot Oct 2022 #9
Nine is just the Sacred Three three times. niyad Oct 2022 #16
And there definitely is that! calimary Oct 2022 #24
yes Skittles Oct 2022 #11
and the sum 9 packman Oct 2022 #12
Not the 9s trick, but I remember doing paperbacks w/Magic Squares: anyone? Backseat Driver Oct 2022 #14
Yes, I did and I shared it with my students. BigmanPigman Oct 2022 #17
Yep. Multiplication is really just an addition shortcut. paleotn Oct 2022 #23
Shit. That ain't nuthin. I can take off my shoes 3Hotdogs Oct 2022 #25
And I can get to 21 by dropping my shorts packman Oct 2022 #26
-- ain't nuthin, either. Take off my shorts and I gets to 21 and 31/32nds. 3Hotdogs Oct 2022 #28
You're a hell of a man packman Oct 2022 #30
Aw shucks. 3Hotdogs Oct 2022 #31
Lol... name checks out... 3Hotdogs IcyPeas Oct 2022 #36
Never learned this in my public school education - but I got it now. Thanbks for the post. n/t iluvtennis Oct 2022 #27
Now do 8's sarchasm Oct 2022 #32
Of course there were nine in the Fellowship of the Ring. n/t spike jones Oct 2022 #33
IcyPeas .....................Cool! Upthevibe Oct 2022 #34
I feel cheated intrepidity Oct 2022 #39
All digits will always sum to nine. Jimbo S Oct 2022 #43

Chainfire

(17,542 posts)
1. The teachers did not teach us that trick, but we learned it from a joke.
Sun Oct 23, 2022, 05:02 PM
Oct 2022

Our teachers would have considered that "cheating."

calimary

(81,298 posts)
22. Got this one from the nuns!
Sun Oct 23, 2022, 07:29 PM
Oct 2022

I forget which of them pointed it out, in which class, but yeah. Catholic school.

Marthe48

(16,963 posts)
2. 9 is special like that
Sun Oct 23, 2022, 05:21 PM
Oct 2022

any number divisible by 9, no matter how many places, the digits add up to 9 (1,341, ,270, 81,000 for example) If you add the digits of 99, you get 18, which equals 9.

calimary

(81,298 posts)
21. The "nines" were always easiest for me.
Sun Oct 23, 2022, 07:16 PM
Oct 2022

I loved how that all worked, from top to bottom. Always nine.

Staph

(6,251 posts)
38. Three works, too, sort of!
Sun Oct 23, 2022, 11:21 PM
Oct 2022

With any number divisible by 3, if you add the digits, and then add the digits of the answer, ad infinitum, the answer will always be 3 (or a single digit number divisible by 3, like 6 and 9).

2022 -- 2 + 0 + 2 + 2 = 6!


Marthe48

(16,963 posts)
41. 3 is another number that is special
Mon Oct 24, 2022, 09:19 AM
Oct 2022

Partly, because it is the sq. root of 9. I used to know a lot more about fun with numbers. My great aunt had several fortune-telling games based on numerolgy, maybe that's what got me interested

LetMyPeopleVote

(145,291 posts)
4. Yes
Sun Oct 23, 2022, 05:27 PM
Oct 2022

Some of this I learned in class and some while competing in a contest called number sense where you did math problems in your head (no scratch pads).

TexasTowelie

(112,217 posts)
10. All of the number sense tricks come in handy,
Sun Oct 23, 2022, 06:03 PM
Oct 2022

particularly when conducting audits and reconciliations as I did throughout my career.

niyad

(113,323 posts)
15. I cannot believe how many people I taught this little reconciliation trick to:
Sun Oct 23, 2022, 06:55 PM
Oct 2022

If the difference in the two amounts is divisible by 9, there is a transposition error, and the number of digits in the answer tells you which columns.

teach1st

(5,935 posts)
6. I taught it for years in elementary
Sun Oct 23, 2022, 05:41 PM
Oct 2022

The real trick, though, is letting the kids work in small groups to figure out why the pattern occurs and why only with nine. The small groups thing might not work in third grade, but it has worked - and been unsuccessful at times - in fourth and fifth grades.

wnylib

(21,479 posts)
29. I wish that I had been taught math like that.
Sun Oct 23, 2022, 07:57 PM
Oct 2022

Instead, I was taught by rote. Fortunately, I have a good memory, but the rote method did nothing to develop any real understanding of numbers

In grade school, the teacher put addition "problems" across the top of the blackboard. For about a month in second grade, it would be 1+ the rest of the numbers up to 9, and 2+ numbers to 9 in a row underneath, followed by 3+ numbers to 9. Then those were erased and replaced with 4, 5, and 6 + numbers to 9 for another month, etc.

Every time we did anything that required us to write on paper, we had to first copy all of the rows on the blackboard. After addition came subtraction in the same method.

Then in third grade, it was multiplication and division, but up to 12. Fouth grade was review, with 10 through 12 added to the addition and subtraction. Fifth grade was long division.

There was no mention of number sets or of prime numbers, just rote memorization.

So arithmetic was something I learned to do and to use when necessary, but it was boring. Math did not get interesting to me until we were introduced to elementary algebra in 9th grade.



mitch96

(13,907 posts)
35. "I was taught by rote. Fortunately, I have a good memory" Me too but I have a crap memory.
Sun Oct 23, 2022, 08:52 PM
Oct 2022

I still have a problem with multiplication tables.. Formulas? Those I can work with and use a calculator. Needed formulas for work and I loved my work so I made it work...
Math always gave me a brain cramp.. What everybody "got" I did not. For a while I though I was a product of some alien sperm that did not have a "math" gene...
m

wnylib

(21,479 posts)
37. LOL. My father was so good at mental calculations
Sun Oct 23, 2022, 09:38 PM
Oct 2022

in math that he was like a human computer. My oldest brother was also a mathematical whiz all through school and into college. My younger sister started out as a math major in college because she was so good at it, but then got interested in sociology and switched majors.

That math gene skipped me. I did not hate it in school, but was bored with the way that arithmetic was taught in grade school so I tuned it out. In a family with 3 math whizzes, I felt inadequate in it. My good memory worked well in language study, which was much more interesting to me because of the variety of cultures that languages introduce people to.

But I did get more interested in math once I had algebra and geometry in high school. I later realized that I was not actually bad at math when I had a job that required me to do some quick, simple algebra and geometry calculations in my head. I just did not have the natural aptitude for math that other people I knew had.

My aptitude was languages and literature. So I majored in languages and minored in anthropology.


mitch96

(13,907 posts)
40. "languages and literature." I'll take Language and Lit any day of the week over "the maths"
Mon Oct 24, 2022, 07:58 AM
Oct 2022

Language always fascinated me. I had to learn a bit of spanish b/c of where I worked. Miami.
The nuances were a hoot. The same word in one country means or indicates something else in another country but it's still "Spanish"...I would have loved have taken a spanish language courses in different country's. Madrid, Mexico City, Ecuador.. I have been told I have a good ear for accents. A parrot.
Patients thought I was a native speaker b/c I spoke Spanish with a "cuban" accent... even though I sometimes did not know what I was saying.
I would get some words or phrases mixed up.. Like De Nada and De Nalgas..
Going to a foreign country, speaking the language and eating their food..
Really get to know them.. Now THAT would be a trip..
m

wnylib

(21,479 posts)
42. I have studied Latin, German, and Spanish.
Mon Oct 24, 2022, 10:41 AM
Oct 2022

Spanish was my major in college. I taught high school and middle school Spanish for a while, but most of my experience was teaching English to adult English language learners. The vast majority of them had Spanish as their native language. They were from all over Latin America - Puerto Rico, Mexico, Colombia, Ecuador, Venezuela, Costa Rica, Cuba, Dominican Republic, and Peru. One was Italian and had lived in Germany for a while. I guess that two languages, Italian and German, were his limit. He had a very hard time with English and kept putting Italian and German words and phrases into his English. One woman was Portuguese and her husband was Dutch.

Several of my students became personal friends after their classes were finished. They were homesick and a large group of them became each other's "family" away from home, with Spanish as a common denominator, despite the differences in accents, idioms, and some words. I was the "honorary Mexican" because they made up the largest number in the group and my vocabulary was more Mexican than any other form of Spanish.

We got together as a group for Christmas and birthdays, and in summer for picnics, plus other occasions like children's graduations, weddings, etc. Their church had mass in Spanish and a few times I went with them to a church event. One time the priest (from Colombia) asked me where I was from because my Spanish sounded "authentic" to him but he could not place the accent. I said, "Soy la gringa del grupo. Soy de Pennsylvania."

We have drifted apart over the years. Children are grown. Some people were transferred to other places in their jobs. A few have passed away. But at one point, we numbered about 30, plus spouses or boyfriends and children. When we went out for dinner (without the children) we took up 2 banquet tables, jabbering away in English, Spanish, and Spanglish.

There were some hilarious moments of misunderstanding, using a wrong word or phrase, them in English and me in Spanish. And some funny moments when they learned English words for things. For example, we had dinner at a Mexican friend's house who invited a newcomer from Mexico whose English was very limited at the time. The dessert was angel food cake. The newcomer said it was delicious and asked what it was called. She thought we were putting her on when we said angel food. So the hostess, also Mexican, pulled a devil's food cake mix out of the cupboard and told her that there is devil's food in America, too, but only for bad people. The look on the newcomer's face was priceless.



skypilot

(8,854 posts)
9. A bit off topic but..
Sun Oct 23, 2022, 05:58 PM
Oct 2022

...the number 9 is the one thing I'm truly superstitious about. I won't go into all the history of it but I will not live in a house or apartment building where the numbers in the address add up to 9.

Backseat Driver

(4,392 posts)
14. Not the 9s trick, but I remember doing paperbacks w/Magic Squares: anyone?
Sun Oct 23, 2022, 06:49 PM
Oct 2022

I found math puzzling and difficult.

Our elementary school was a guinea pig for the New Math - cheap paper mimeographed workbooks directly printed w/staple bindery out of the Yale printshop, I think. They weren't much into the tricks - used the long, involved processes instead of rules as I recall - endlessly circling 3 groups of 2 and 2 groups of 3 for proof they both equaled 6; thankfully, much modified since then...

Dear Bro learned to read by the whole word method; for him--not so good; he had a lot of problems. We learned phonics...

BigmanPigman

(51,607 posts)
17. Yes, I did and I shared it with my students.
Sun Oct 23, 2022, 07:03 PM
Oct 2022

Also, if you put both hands out with fingers extended use the hands and fingers for the place of the digits. It only works for the "nines" though. My sister showed me this trick in my 30s! Why didn't she tell me this when I needed it in my own math class?!?

For example, 9 times 1...

Put down your pinky finger on your left hand for the ones place (9 times 1) and look at both hands. The left hand has 4 fingers and the right has 5 fingers so the place value is 4 plus 5 which is 9 for the "ones" place. Then for 9 times 2 put down the second from the left finger, the "2" on your left hand and you see 1 finger (place value 1) on the left of it and there are 3 more fingers to the right of it on the left hand and 5 on the right (3 plus 5 is 8). So you have "1" in the "tens" place and 8 in the "ones" place making "18".

Then for 9 times 3 do the same...put the third finger on your left hand down a d to the right of it you have 2 fingers and to the right you have 2 remaining up on the left hand and 5 on the right hand. The 2 fingers are the tens place of 2 and the remaining fingers are equal to 7 making it 27.

It is hard to explain and you really need the visuals. I taught this to my 1st graders and it empowered them with their older siblings who didn't know the trick.

I learned during my years teaching in elementary school that math is mainly patterns. It is all about patterns. Once you know this it makes math so much easier and even fun. The 1st graders actually loved math and their parents were amazed.

paleotn

(17,920 posts)
23. Yep. Multiplication is really just an addition shortcut.
Sun Oct 23, 2022, 07:29 PM
Oct 2022

Mathematics is full of wonderful symmetry. One of the things I've always loved about it.

intrepidity

(7,302 posts)
39. I feel cheated
Mon Oct 24, 2022, 05:52 AM
Oct 2022

As for others above, arithmatic was taught as pure rote memorization. I never enjoyed math until geometry, because then I could visualize it. Many life/career decisions would later be based on this weak math foundation. Only much, much later in life (up to and including now) have I begun to learn to enjoy and appreciate math.

Grrrrrrr

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