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Why wasn't I ever taught this?! (Warning: Math inside thread)


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#1 Mainly Mattias

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Posted 15 January 2013 - 04:00 AM

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This seems so bloody easy! ..not to mention quick!
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#2 n00bxQb

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Posted 15 January 2013 - 05:27 AM

My grade 8 teacher taught me that. I guess that would've been 16 years ago.

Edited by n00bxQb, 15 January 2013 - 06:11 AM.

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#3 Rounoush

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Posted 15 January 2013 - 06:31 AM

...Okay, I'm just gonna say it: How does that work?
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#4 n00bxQb

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Posted 15 January 2013 - 06:37 AM

...Okay, I'm just gonna say it: How does that work?

It's pretty simple.

Take the first number (1 from 13) and draw a line. Below that, leaving some space between, draw a number of lines equal to the second number (3 from 13). Now do the same for the multiplier, but perpendicular.

Separate the different spaces and count the intersections within those spaces. There is 1 intersection in the top-left corner, 5 intersections in the middle section, and 6 intersections in the bottom-right corner.

See video:
http://www.youtube.com/watch?v=e-P5RGdjICo

As for quickness, there are quicker ways, especially on 2 digit x 2 digit multiplications.

Edited by n00bxQb, 15 January 2013 - 06:38 AM.

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#5 nucklehead

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Posted 15 January 2013 - 07:15 AM

As for quickness, there are quicker ways, especially on 2 digit x 2 digit multiplications.


I think I solved it in my head twice before he was done
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#6 jmfaminoff

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Posted 15 January 2013 - 08:17 AM

I think I solved it in my head twice before he was done

We used to rehearse the tables by memory. Because 12X12 is 144, 12X13 is 144+12=156
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#7 avelanch

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Posted 15 January 2013 - 08:52 AM

i would think that this would be stupidly slow for anything with more than 5 lines.

or single digits.

9x9, for example, counting 81 intersections? no thanks.
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#8 D-Money

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Posted 15 January 2013 - 08:57 AM

That probably works well for kids who respond well to visual aids.

Me, I just always liked numbers, and never had a problem thinking in them.
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#9 qball

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Posted 15 January 2013 - 10:00 AM

Mathematical models are great for helping kids to see the 'big picture' (what's really going on when we do stuff with numbers), but at the same time, they're rarely practical or efficient enough for general use...
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#10 Heretic

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Posted 15 January 2013 - 10:14 AM

That probably works well for kids who respond well to visual aids.

Me, I just always liked numbers, and never had a problem thinking in them.


Same.

I do it this way in my head:

13 x 12 = 13 x 10 + 13 x 2
13 x 12 = 130 + 26
13 x 12 = 156
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#11 Mountain Man

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Posted 15 January 2013 - 10:15 AM

Calculators have always worked for me.
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#12 GodzillaDeuce

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Posted 15 January 2013 - 10:19 AM

56 = 7 * 8

5, 6, 7, 8!

Divisibility by 3: add up digits and check result for divisibility by 3. eg. 10,407 is divisible by 3 since 1+0+4+0+7=12 is divisible by 3

those are my math tricks lol
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well I'm sorry that gd is soo perfect


#13 GodzillaDeuce

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Posted 15 January 2013 - 10:20 AM

Same.

I do it this way in my head:

13 x 12 = 13 x 10 + 13 x 2
13 x 12 = 130 + 26
13 x 12 = 156


same. just round one number to nearest 5 then adjust accordingly
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well I'm sorry that gd is soo perfect


#14 D-Money

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Posted 15 January 2013 - 11:09 AM

This isn't really a trick, but is fascinating to me:

Multiply any number by 9, then add the digits together. They will always equal 9, or a multiple thereof (18, 27, etc.)

9x2 = 18 ; 1+8 = 9
9x6 = 54 ; 5+4 = 9
9x17 = 153 ; 1+5+3 = 9
9x54 = 486 ; 4+8+6 = 18
9x238 = 2,142 ; 2+1+4+2 = 9
9x6,841 = 61,569 ; 6+1+5+6+9 = 27
9x483,902 = 4,355,118 ; 4+3+5+5+1+1+8 = 27
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#15 Mainly Mattias

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Posted 15 January 2013 - 02:43 PM

56 = 7 * 8

5, 6, 7, 8!

Divisibility by 3: add up digits and check result for divisibility by 3. eg. 10,407 is divisible by 3 since 1+0+4+0+7=12 is divisible by 3

those are my math tricks lol

interesting! i'll remember that. don't know when i'd ever need it mind you..
maybe when splitting a dinner tab but i'd still prob end up whipping out the phone calc. lol
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#16 nucklehead

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Posted 15 January 2013 - 05:24 PM

interesting!  i'll remember that.  don't know when i'd ever need it mind you..
maybe when splitting a dinner tab but i'd still prob end up whipping out the phone calc. lol


You should never whip it out at the table.Bad luck.
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#17 -Vintage Canuck-

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Posted 15 January 2013 - 05:33 PM

I like how it says "(Warning: Math inside thread)". :lol:
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#18 avelanch

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Posted 15 January 2013 - 06:08 PM

This isn't really a trick, but is fascinating to me:

Multiply any number by 9, then add the digits together. They will always equal 9, or a multiple thereof (18, 27, etc.)

9x2 = 18 ; 1+8 = 9
9x6 = 54 ; 5+4 = 9
9x17 = 153 ; 1+5+3 = 9
9x54 = 486 ; 4+8+6 = 18
9x238 = 2,142 ; 2+1+4+2 = 9
9x6,841 = 61,569 ; 6+1+5+6+9 = 27
9x483,902 = 4,355,118 ; 4+3+5+5+1+1+8 = 27

the real mindblowing thing is that if you have double digits in the first answer, add them together again and it'll add up to 9.:
9x54 = 486 ; 4+8+6 = 18 ; 1+8 = 9
9x6,841 = 61,569 ; 6+1+5+6+9 = 27 ; 2 + 7 = 9
9x483,902 = 4,355,118 ; 4+3+5+5+1+1+8 = 27 ; 2 + 7 = 9
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#19 Mainly Mattias

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Posted 15 January 2013 - 09:26 PM

You should never whip it out at the table.Bad luck.

:bigblush:
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#20 Evolution10

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Posted 16 January 2013 - 01:01 AM

I like how it says "(Warning: Math inside thread)". :lol:


Good for people like me who aren't big fans of math :lol:


On the otherhand, some of the posts in this topic are really neat. Learned something new today :)
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#21 That's What She Said

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Posted 16 January 2013 - 01:42 AM

This isn't really a trick, but is fascinating to me:

Multiply any number by 9, then add the digits together. They will always equal 9, or a multiple thereof (18, 27, etc.)

9x2 = 18 ; 1+8 = 9
9x6 = 54 ; 5+4 = 9
9x17 = 153 ; 1+5+3 = 9
9x54 = 486 ; 4+8+6 = 18
9x238 = 2,142 ; 2+1+4+2 = 9
9x6,841 = 61,569 ; 6+1+5+6+9 = 27
9x483,902 = 4,355,118 ; 4+3+5+5+1+1+8 = 27

Hadn't heard of the method in the OP but I thought this was common knowledge. Do they not teach it in high school anymore when they cover reducing fractions? Sure makes it a lot faster to figure out the greatest common factor.
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