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I'm stuck on the April 5 puzzle

 
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Bill Denholm



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

PostPosted: Sat Apr 08, 2006 2:30 am    Post subject: I'm stuck on the April 5 puzzle Reply with quote

I got stuck fairly early. I tried the site's Play option for the first time to get a hint. It told me to play 5 at (R8,C1) but I don't understand the logic to the move. I had located my 5s in row 7 at either (R7,C2) or (R7,C7) and my 5s in row 8 at either (R8,C1) (the hint) or (R8,C7). Can anybody explain why the hint is a logical move?

Bill
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keith



Joined: 19 Sep 2005
Posts: 3136
Location: near Detroit, Michigan, USA

PostPosted: Sat Apr 08, 2006 12:03 pm    Post subject: Pairs and intersections Reply with quote

There is a pair <26> in R9 that removes <6> as a possibility in R8C1. Then you can hunt down another pair <48> in R8, so <5> is the only possible value in R8C1.

Keith
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PostPosted: Sat Apr 08, 2006 12:19 pm    Post subject: Reply with quote

Bill,
The second grid reflects the candidate list after placing the naked and hidden singles at the start of the puzzle, which I'm assuming is where you got stuck.

Code:

 *-----------*
 |9.3|...|6..|
 |...|..5|.2.|
 |...|..1|9.8|
 |---+---+---|
 |..9|.57|...|
 |..5|3.4|1..|
 |...|69.|7..|
 |---+---+---|
 |3.1|7..|...|
 |.9.|2..|...|
 |..7|...|8.4|
 *-----------*
 *--------------------------------------------------------------------*
 | 9      145    3      | 8      7      2      | 6      145    15     |
 | 14678  14678  468    | 9      36     5      | 34     2      137    |
 | 2567   2567   26     | 4      36     1      | 9      357    8      |
 |----------------------+----------------------+----------------------|
 | 2468   23468  9      | 1      5      7      | 234    3468   236    |
 | 678    678    5      | 3      2      4      | 1      689    69     |
 | 124    1234   24     | 6      9      8      | 7      345    235    |
 |----------------------+----------------------+----------------------|
 | 3      24568  1      | 7      468    69     | 25     569    2569   |
 | 4568   9      468    | 2      13468  36     | 35     13567  13567  |
 | 26     26     7      | 5      136    369    | 8      1369   4      |
 *--------------------------------------------------------------------*


In addition to the naked pair mentioned by keith in row 9, there is also one in column 5. When the exclusions for both these are made, that leads to the other pair mentioned by keith the one in row 8. Making the exclusions from all the pairs leaves 5 as the only possibility for r8c1.
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Bill Denholm



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

PostPosted: Sat Apr 08, 2006 2:34 pm    Post subject: Reply with quote

Thanks to both of you. I am new to SuDoku and to this forum. It was the 48 pairs at (R7,C5) and (R8,C5) that I didn't realize I could use.

Bill
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