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dejsmith



Joined: 23 Oct 2005
Posts: 40

PostPosted: Sun Oct 23, 2005 9:06 pm    Post subject: Please Help Reply with quote

I am new to Sudoku, but have been learning by reading many of your helpful suggestions. I may have gone over my head with one too difficult for me at this stage, though. Following is the initial setup:

--8 -9- ---
-5- 82- 7--
--- --1 -6-
3-- --- -7-
7-2 3-4 8-1
-1- --- --5
-7- 4-- ---
--4 -62 -8-
--- -8- 6--

I think I am correct to the following point, where I could go no farther:

-48 79- ---
-5- 82- 7--
--7 541 -68
385 219 476
762 354 891
419 678 --5
876 435 ---
--4 -62 -87
--- -87 6--

Any technique hints would be appreciated. Thanks.
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David Bryant



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

PostPosted: Sun Oct 23, 2005 10:25 pm    Post subject: This is an "other" puzzle Reply with quote

This same puzzle came up for discussion yesterday in the "Other Puzzles" section of this forum. You can link to that discussion here.

You got stuck a little bit sooner than Louise56 did. What you need to notice is that the value "3" can only fit in two places in row 3 -- either at r3c2 or at r3c7. Similarly, the value "3" only has two places where it can fit in row 8 -- either at r8c2 or at r8c7. This forms the famous "X-Wing" formation -- since these four cells lie at the corners of a rectangle, we can be certain that either r3c2 = 3 and r8c7 = 3, or else r8c2 = 3 and r3c7 = 3. There's simply no other way the "3"s can fit in these two rows.

Using this insight we can eliminate all other "possible" occurrences of the value "3" in columns 2 & 7 because they are in fact not possible. In particular you can then place a "2" in r6c7 and a "3" in r6c8.

From there you're in the same spot Louise56 got to yesterday -- please refer to that discussion to learn about the "XY-Wing" technique that's needed to finish this puzzle. dcb Very Happy
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dejsmith



Joined: 23 Oct 2005
Posts: 40

PostPosted: Mon Oct 24, 2005 2:26 am    Post subject: Reply with quote

Sorry to make you post twice for the same puzzle. I'll be more careful to search the whole site before asking another question. Thanks for your help!
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