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 Post subject: A symmetry question
PostPosted: Tue Dec 20, 2011 9:09 am 
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Joined: Sat May 28, 2011 2:26 pm
Posts: 10
Dear All

Could anyone out help in resolving a question that my niece and I are having a disagreement with ?

The question is :

A square grid is separated into 16 small squares ( a diagram of a 4 by 4 square is given but I can't seem to insert diagrams here - each corner is identified as A B C and D and diagonal line from AC )
How many ways can two squares be shaded so that the grid has symmetry about the diagonal AC if :

a) the two squares must not include the diagonal line ?

b) the two squares must include the diagonal line ?

c) On the grids below ( again six 4 by 4 squares are given with A B C D as the corners with two diagonal lines AC and BD),
show all the ways exactly two squares can be shaded so that the grid has symmetry about the two diagonal lines AC and BD. The question also says - (You may not need to use all the grids).


If hermanmunster or anyone else out there can help - then we would be very grateful and would like to thank you in advance :)


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PostPosted: Tue Dec 20, 2011 10:07 am 
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Joined: Thu Jan 03, 2008 8:26 am
Posts: 1324
Location: Watford, Herts
teacher_mum wrote:
A square grid is separated into 16 small squares ( a diagram of a 4 by 4 square is given but I can't seem to insert diagrams here - each corner is identified as A B C and D and diagonal line from AC )
How many ways can two squares be shaded so that the grid has symmetry about the diagonal AC if :

a) the two squares must not include the diagonal line ?

b) the two squares must include the diagonal line ?

Code:
  A _____________ B
    |__|__|__|__|
    |__|__|__|__|
    |__|__|__|__|
    |__|__|__|__|
  D               C

a) One of the shaded squares must be below the diagonal and the other above. There are 6 possibilities for the first square, and once we've chosen that the other is fixed by the symmetry, so 6 possible answers.
b) If one of the squares is on the diagonal, the other must be as well to obtain symmetry, so the question becomes how many ways can we choose 2 squares out of the 4 on the diagonal. Well, there are 4 ways to choose the first square, times 3 to choose the second (once one square is taken), divided by 2 because we've considered each combination twice, so again 6 possible answers.


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 Post subject: Re: A symmetry question
PostPosted: Thu Dec 22, 2011 9:39 am 
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Joined: Sat May 28, 2011 2:26 pm
Posts: 10
Dear WP

Thank you very much for your help :)

I had 6 for part (i) but 4 for (ii) but you are right it should 6 as well.

Thanks once again :)


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 Post subject: Re: A symmetry question
PostPosted: Mon Jun 25, 2012 12:20 pm 
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Joined: Mon Jun 25, 2012 12:14 pm
Posts: 1
Hi There

For part c) for symmetry around 2 axes, my answer was 4 possibilities - could someone please confirm if this is correct?

Thanks


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