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help solving

 
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wood2wn



Joined: 26 Oct 2005
Posts: 1
Location: Denmark

PostPosted: Wed Oct 26, 2005 7:20 pm    Post subject: help solving Reply with quote

When I fill in the blank grid I get a hint, but I can't figure out the logic.

9 - - 6 - - - 5 8
8 - 4 - - - 6 1 -
6 2 - - 8 - - 3 -
2 - - 7 - 3 5 - 4
7 - 6 - 5 - 1 - 3
3 - - 1 - 8 - - -
4 7 8 3 1 6 9 2 5
5 6 2 8 - - 3 7 1
1 9 3 5 2 7 8 4 6

The tip I get is is 7 in first row just before 6 , and I canīt figure out why.
Can anyone please tell me why?
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David Bryant



Joined: 29 Jul 2005
Posts: 559
Location: Denver, Colorado

PostPosted: Thu Oct 27, 2005 11:51 am    Post subject: Why a "7" in r1c3? Reply with quote

Hi, wood2wn! Welcome to the forum.

The answer you seek lies in row 3.

I think it's fairly obvious that the only possibilities for r1c3 are {1, 7}. It's also clear that the only way a "7" can fit in the top left 3x3 box is either at r1c3 or at r3c3.

Let's analyze row 3. The only possibilities at r3c4 are {4, 9}. The only possibilities at r3c7 are {4, 7}. The only possibilities at r3c9 are {7, 9}. Therefore the values {4, 7, 9} must occupy the cells r3c4, r3c7, & r3c9 in some order ("naked triplet"), leaving the values {1, 5} to occupy the two cells r3c3 & r3c6.

Therefore the "7" must go in r1c3, because it can't be placed at r3c3. dcb Smile
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