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Sept 16 - very hard

 
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Bill Denholm



Joined: 24 Mar 2006
Posts: 17
Location: Mountain View, California

PostPosted: Sun Sep 17, 2006 2:03 am    Post subject: Sept 16 - very hard Reply with quote

I got stuck and asked for a hint. The program gave me 5 in R5C9. I found the UR (X-Wing) but I thought that only let me eliminate 1s inside of the rectangle. Is this correct? Otherwise, what is the logic behind the hint?

Bill
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David



Joined: 02 Jun 2006
Posts: 58
Location: Bedford, UK

PostPosted: Sun Sep 17, 2006 8:51 am    Post subject: Reply with quote

Hi Bill,

Do you mean the hint was row 9 column 5 ?
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Marty R.



Joined: 12 Feb 2006
Posts: 5176
Location: Rochester, NY, USA

PostPosted: Sun Sep 17, 2006 3:53 pm    Post subject: Reply with quote

An X-Wing is not the same as a UR. With an X-Wing on columns, you can eliminate the number from all other rows in the two columns, and eliminate from all other columns with an X-Wing on rows.

I did this puzzle with pencil and paper. After the basic moves, there was an X-Wing on "1" in columns and the eliminations therefrom allowed completion of the puzzle with basic techniques.

Here's a good explanation of X-Wings. Scroll down about halfway to find it.

http://www.angusj.com/sudoku/hints.php
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Bill Denholm



Joined: 24 Mar 2006
Posts: 17
Location: Mountain View, California

PostPosted: Sun Sep 17, 2006 5:28 pm    Post subject: Reply with quote

Yes, that was what I meant. <g> I got them switched.

Bill
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Bill Denholm



Joined: 24 Mar 2006
Posts: 17
Location: Mountain View, California

PostPosted: Sun Sep 17, 2006 5:32 pm    Post subject: Reply with quote

Marty,

Thanks. I guess I don't understand the difference between an X-Wing and a Unique Rectangel (UR), if there is one.

Bill
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