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Posted: Sat Oct 01, 2005 6:41 pm Post subject: oct1's very hard puzzle 


stuck again
57x 39x 182
x3x xxx x54
x8x 5xx 63x
xx2 4x5 xx3
x5x xx3 2xx
xx3 629 5xx
x25 xxx 39x
3xx 95x x2x
x91 x3x x75 

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

Posted: Sat Oct 01, 2005 7:47 pm Post subject: Spotting pairs and triplets 


Your next logical move is to place an "8" at r4c5. I'll explain why as best I can.
First look at the upper left 3x3 box. Because of the "2" at r4c3 and the "1" at r9c3 we can deduce that the cells r2c1 & r3c1 must contain the pair {1, 2}. Therefore the cells r1c3, r2c3, and r3c3 (third column in upper left 3x3 box) must contain the triplet {4, 6, 9}. By elimination, then, the pair {7, 8} must appear in r5c3 & r8c3.
Now look at the 6th row. The only missing values are {1, 4, 7, 8}. Since the pair {7, 8} already appears in column 2 and in column 8 we can locate the pair {1, 4} in r6c2 and r6c8. By elimination, then, the pair {7, 8} must appear at r6c1 & r6c9.
But now we have the pair {7, 8} in the middle left 3x3 box  in r6c1 and r5c3. In particular this means that an "8" cannot appear in either r4c1 or r4c2.
Now look at the bottom right 3x3 box. We have a similar pattern  the "1" at r1c6 and the "6" at r3c6 tell us that the pair {1, 6} must appear in r7c9 and r8c9  by elimination the pair {4, 8} must appear in r8c7 and r9c7. This implies that the "8" in the middle right 3x3 box must appear in either r5c9 or in r6c9, since there's already an "8" at r1c8.
But this implies that "8" cannot appear in either r4c7 or r4c8, leaving r4c5 as the only cell in the fourth row in which the "8" can be placed.
Hopefully you can solve the puzzle from there  this one's not so very hard, once you spot a few other pairs and triplets besides the few I mentioned. For instance, once you place the "8" at r4c5 you will spot the pair {1, 7} in r5c4 and r5c5, implying that r5c3 must contain an "8", and so forth ... dcb 

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Guest

Posted: Sat Oct 01, 2005 8:37 pm Post subject: 


thx blake
where I got stuck is the '6' in row 3, now I can easily put the '8' into r4c5.
what I need is more practice. the most interesting playing sudoku is like that, a breakthrough can help you solve the puzzle. 

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aakif
Joined: 05 Sep 2005 Posts: 2

Posted: Mon Oct 03, 2005 9:10 am Post subject: Oct 1 Very Hard Puzzle 


I was stuck in the same position as above. Then I draw the puzzle and get the hint to place a No. 9 in R4C1. Still I not figure out the logic ????. Can anyone explain ? 

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someone_somewhere
Joined: 07 Aug 2005 Posts: 275 Location: Munich

Posted: Mon Oct 03, 2005 1:21 pm Post subject: 


Hi Aakif,
You can follow the logic:
1 not in r4c1, it is in r4c2 or r6c2 (Column on 3x3 Block interaction)
1 not in r5c1, it is in r4c2 or r6c2 (Column on 3x3 Block interaction)
1 not in r6c1, it is in r4c2 or r6c2 (Column on 3x3 Block interaction)
1 not in r5c9, it is in r4c8 or r5c8 or r6c8 (Column on 3x3 Block interaction)
1 not in r6c9, it is in r4c8 or r5c8 or r6c8 (Column on 3x3 Block interaction)
6 not in r5c9, it is in r4c8 or r5c8 (Column on 3x3 Block interaction)
6 not in r4c1, Nacked Pair 1 6 in r4c2 and r4c8 (same Row)
1 not in r4c5, Nacked Pair 1 6 in r4c2 and r4c8 (same Row)
4 not in r6c1, Hidden Pair 7 8 in r6c1 and r6c9 (in Row)
1 not in r5c8, it is in r5c4 or r5c5 of the 3x3 (3x3 Block on Row/Column interaction)
4 not in r3c1, Hidden Pair 1 2 in r2c1 and r3c1 (in Column)
6 not in r2c1, Hidden Pair 1 2 in r2c1 and r3c1 (in Column)
9 not in r2c1, Hidden Pair 1 2 in r2c1 and r3c1 (in Column)
9 not in r3c1, Hidden Pair 1 2 in r2c1 and r3c1 (in Column)
4 not in r5c3, Hidden Pair 7 8 in r5c3 and r8c3 (in Column)
4 not in r8c3, Hidden Pair 7 8 in r5c3 and r8c3 (in Column)
6 not in r5c3, Hidden Pair 7 8 in r5c3 and r8c3 (in Column)
6 not in r8c3, Hidden Pair 7 8 in r5c3 and r8c3 (in Column)
9 not in r5c3, Hidden Pair 7 8 in r5c3 and r8c3 (in Column)
8 not in r4c7, Hidden Pair 7 9 in r2c7 and r4c7 (in Column)
8 not in r7c9, Hidden Pair 1 6 in r7c9 and r8c9 (in Column)
8 not in r8c9, Hidden Pair 1 6 in r7c9 and r8c9 (in Column)
7 not in r5c1, Hidden Pair 4 6 in r5c1 and r5c8 (in Row)
8 not in r5c1, Hidden Pair 4 6 in r5c1 and r5c8 (in Row)
9 not in r5c1, Hidden Pair 4 6 in r5c1 and r5c8 (in Row)
9 in r5c9  Unique Horizontal
9 in r3c3 9 in r4c1  Unique Horizontal
and from here one you can swim alone ...
see u, 

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