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PostPosted: Thu Sep 20, 2012 8:00 am 
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Joined: Thu Sep 20, 2012 7:42 am
Posts: 5
Can anyone help me solve this question please. A four side polygon has two diagonals. If a polygon has 9 diagonals how many sides does it have?
My husband and daughter both came up with 18 (I am rubbish at maths, so didn't even attempt it). According to the marking scheme the answer is 6.

I will be grateful for an axplanation as to how to calculate number of sides from diagonals given and vice versa.

Thank you


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PostPosted: Thu Sep 20, 2012 8:27 am 
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Joined: Fri Oct 12, 2007 12:42 pm
Posts: 3813
Location: Chelmsford and pleased
I get children to draw this one out.

3 sides 0 diagonals
4 sides 2 diagonals
5 sides 5 diagonals
6 sides 9 diagonals

There is a formula but it is beyond most 11 year olds

n(n - 3)/2

n is the number of sides, so for a six sided shape we have
6(6 - 3)/2
6 x 3 / 2
18/2 = 9


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PostPosted: Thu Sep 20, 2012 8:52 am 
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Joined: Sun Sep 09, 2012 8:28 am
Posts: 14
Thanks moved ,I actually thought the question was moved ..lol

@OP just as moved mentioned I think this is one in which they will have to use their memory ,just like they normally do for lines of symmetry ..I've come across that kind of question several times and have never seen one that needed the algebraic formula to solve .It's more about testing their ability to solve simple word problems using knowledge they've acquired over the school years .


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PostPosted: Thu Sep 20, 2012 8:53 am 
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Joined: Mon Aug 22, 2011 8:20 pm
Posts: 1706
Location: Warwickshire
See the explanation posted recently here: http://www.elevenplusexams.co.uk/forum/11plus/viewtopic.php?f=2&t=27517&p=322873#p322873


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PostPosted: Thu Sep 20, 2012 3:08 pm 
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Joined: Thu Sep 20, 2012 7:42 am
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Thank you very much, moved, peter doe and okanagan. I'll pass this on to my husband. :)


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