dailysudoku.com

[dejsmith]

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.

[David Bryant]

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

[dejsmith]

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!