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Mainly Mattias

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

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

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

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

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

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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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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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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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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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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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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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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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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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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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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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