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help requested

 
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dl



Joined: 18 Dec 2008
Posts: 4
PostPosted: Thu Dec 18, 2008 7:43 am    Post subject: help requested Reply with quote
i don't know where to go from here... an explanation of a pattern that suggests a solution method would be appreciated... as well as a description of the method.

Code:

+-------------+------------------+------------------+
| 24  7   136 | 24589 245689 256 | 12359 12589 1368 |
| 9   5   36  | 7     268    1   | 23    4     368  |
| 24  8   16  | 2459  24569  3   | 1259  1259  7    |
+-------------+------------------+------------------+
| 7   169 5   | 129   12369  4   | 8     19    13   |
| 8   169 4   | 159   135679 56  | 13579 1579  2    |
| 3   19  2   | 159   1579   8   | 6     1579  4    |
+-------------+------------------+------------------+
| 15  4   9   | 6     1258   7   | 12    3     18   |
| 16  2   78  | 3     18     9   | 4     1678  5    |
| 156 3   78  | 12458 12458  25  | 127   12678 9    |
+-------------+------------------+------------------+

Play this puzzle online at the Daily Sudoku site
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA
PostPosted: Thu Dec 18, 2008 8:53 am    Post subject: Reply with quote
Hint:
Quote:
X-wing

Bigger hint:
Quote:
Look at R27

BIG Hint:
Quote:
Look at <2>


Next:
Quote:
X-wing on <8>


Keith
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Asellus



Joined: 05 Jun 2007
Posts: 865
Location: Sonoma County, CA, USA
PostPosted: Thu Dec 18, 2008 9:03 am    Post subject: Reply with quote
Hi dl ... and welcome!

(I noticed Keith's post just as I was finishing this one. It explains what he was hinting at.)

Look at the <8>s in row 2 and 7. There are only two <8>s in each of those rows and they occur in the same two columns, c5 and c9. I've marked them below:
Code:
+-------------+--------------------+--------------------+
| 24  7   136 | 24589 #24569-8 256 | 12359 12589 #136-8 |
| 9   5   36  | 7     A268     1   | 23    4     B368   |
| 24  8   16  | 2459   24569   3   | 1259  1259   7     |
+-------------+--------------------+--------------------+
| 7   169 5   | 129    12369   4   | 8     19     13    |
| 8   169 4   | 159    135679  56  | 13579 1579   2     |
| 3   19  2   | 159    1579    8   | 6     1579   4     |
+-------------+--------------------+--------------------+
| 15  4   9   | 6     B1258    7   | 12    3     A18    |
| 16  2   78  | 3     #1-8     9   | 4     1678   5     |
| 156 3   78  | 12458 #1245-8  25  | 127   12678  9     |
+-------------+--------------------+--------------------+

These 4 <8>s form a rectangle and either the diagonally opposite <8>s marked "A" are true or those marked "B" are true. In either case, no other <8>s in columns 5 or 9 can be true and are eliminated, as shown at the cells marked #.

This is called an "X-Wing" because those intersecting diagonals resemble the Star Wars crafts of that name. The same thing works in two columns to make eliminations in two rows. In fact, there is another X-Wing in those same two rows that can also make eliminations. See if you can spot it.

The "Very Hard" puzzles at this site can all be solved using X-Wing, XY-Wing, and/or XYZ-Wing, so those are good places to start. You can read explanations online at several places, such as Sudopedia or here and here.

[Edit to repair grid error.]

Last edited by Asellus on Fri Dec 19, 2008 3:03 am; edited 1 time in total
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dl



Joined: 18 Dec 2008
Posts: 4
PostPosted: Thu Dec 18, 2008 9:56 pm    Post subject: Reply with quote
Asellus -

Thanks for the quick response.

I follow the logic of your explanation... but am unsure of the diagram.

Should r7c5 be marked with a "B" and the <8> removed from r8c5?


BTW, Keith's hints aren't visible... looks as though the text in the quote blocks is coded as "white"... tough to see against a white background (may just be the way my browser is configured).
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Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
PostPosted: Thu Dec 18, 2008 10:25 pm    Post subject: Reply with quote
Dl,

To see Keith's hints, just select (highlight) inside the quote box. People do that when they think others may want to try things out and not see the answer.

I do think Asellus got the "B"in the wrong cell.
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Asellus



Joined: 05 Jun 2007
Posts: 865
Location: Sonoma County, CA, USA
PostPosted: Fri Dec 19, 2008 3:04 am    Post subject: Reply with quote
Quote:
I do think Asellus got the "B"in the wrong cell.

Yes I did! Embarassed Sorry about that. I have fixed it.
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