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PostPosted: Tue Oct 23, 2012 1:52 pm 
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Joined: Thu Oct 18, 2012 9:41 am
Posts: 434
Hi ForumMembers.. I am trying to teach DC long multiplication, school teach the box method which I find mind boggling :roll:

If this was the question, how would you teach it, here's my approach. Please comment/correct ... is there an easier way.

Feedback greatly appreciated.

213
45 *
------
0

first cover the 5, then put a zero in, the units coloum

then do 4 * 3 = 12 , put the 2 in carry the 1

213
45 *
------
20
------
1

then do 4 * 1 = 4 (add the 1) = 5

213
45 *
------
520
------
1

now do 2 * 4 = 8

213
45 *
------
8520
------
1

now do the units side

5 * 3 = 15 put the 5 in carry the 1

213
45 *
------
8520
5
--------

then do 5 * 1 = 5 add 1 = 6

213
45 *
------
8520
65
--------

then do 5 * 2 = 10


213
45 *
------
8520
1065
--------


now add together

9585 total

Thanks
SleepyHead


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PostPosted: Tue Oct 23, 2012 7:12 pm 
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Joined: Fri Oct 12, 2007 12:42 pm
Posts: 3810
Location: Chelmsford and pleased
I would teach a grid method as it is useful for multiplying algebra in ks3 and 4. If. A child used another form of algorithm confidently I would be happy for him/her to use it.

Cannot reproduce here, but put 200, 10 and three across the top and 40 and 5 down the sides.

2 x 4 = 8 then add the zeros, in this case 000, etc.

I have used this method with lower attainers at gcse who have never previously grasped long multiplication.

(x + 2)(x + 3)

x and 2 across the top, x and 3 down the side.

Some primary children don't like the method, because they have already learnt the algorithm, if they are happy using it then fine with me.


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PostPosted: Tue Oct 23, 2012 7:23 pm 
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Joined: Mon Aug 22, 2011 8:20 pm
Posts: 1706
Location: Warwickshire
Asked ds how he'd work out 213*45

add a 0 -> 2130 is 213*10
half it -> 1065 is 213 *5
add another zero -> 10650 is 213*50
subtract the 1065 from the 10650.

Doesn't work as well for all numbers of course!


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PostPosted: Tue Oct 23, 2012 8:05 pm 
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Joined: Mon Jun 18, 2007 2:32 pm
Posts: 6963
Location: East Kent
The grid method is surprisingly handy once you get used to it. It makes it easy to spot mistakes, even though I was brought up on "proper" long multiplication I like it


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PostPosted: Tue Oct 23, 2012 10:03 pm 
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Joined: Thu Oct 18, 2012 9:41 am
Posts: 434
Thanks everyone for your replies. That helps clears things up. :)

Grid method is it.. out of interest do schools teach long multiplication method as well?
SleepyHead


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PostPosted: Wed Oct 24, 2012 6:36 am 
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Location: East Kent
start moving towards that year 6 ish.


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PostPosted: Wed Oct 24, 2012 11:43 am 
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thank you.


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PostPosted: Wed Oct 24, 2012 11:45 am 
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Joined: Mon Feb 12, 2007 1:21 pm
Posts: 11934
You don't ever need more than the grid method as we build on that in Secondary and we use it right through GCSE and A level algebra!


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PostPosted: Wed Oct 24, 2012 9:04 pm 
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Joined: Thu Oct 23, 2008 10:07 pm
Posts: 501
Guest55 wrote:
You don't ever need more than the grid method as we build on that in Secondary and we use it right through GCSE and A level algebra!


I'm going to quote this next time I'm told it's a stupid method that takes far too long!


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PostPosted: Wed Oct 24, 2012 9:11 pm 
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Joined: Fri Oct 21, 2011 2:49 pm
Posts: 261
it's much easier to see where a mistake has been made too (if it has)


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