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Need help understanding drawer's tip

 
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PostPosted: Wed Aug 24, 2005 7:50 pm    Post subject: Need help understanding drawer's tip Reply with quote

I found this little gem at Boldt's:
Code:
... ... ...
..8 .7. 3..
.7. 5.8 .6.

..5 .2. 9..
.6. 9.3 .7.
..2 .4. 6..

.1. 8.7 .9.
..3 .5. 4..
... ... ...
Notice the empty border and the nice pattern.
After doing my best, I end up with:
Code:
... ... 7..
..8 .7. 3..
.7* 5.8 16.

..5 .2. 9..
.6. 983 .7.
..2 .45 6..

.1. 8.7 .9.
..3 .5. 4.7
..7 ... 8..
At the * I have marked 4 and 9 as possibles, and this site's drawer gives the tip that it should be a 9. I cannot see how I can eliminate the 4.

Anyone care to enlighten me?
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chobans



Joined: 21 Aug 2005
Posts: 39

PostPosted: Wed Aug 24, 2005 9:09 pm    Post subject: Reply with quote

r4c2 can be 3,4,8
but r5c1 and r5c3 both share two same numbers: 1,4. So 1 and 4 can be eliminated from box 4 (r4c1-r6c3). So r4c2 can only be 3,8.

r9c2 can be 2,4,5,9
but in box 8 (r7c4-r9c6), 4 can ONLY go in row 9. So 4 can be eliminated from other cells in row 9. So r9c2 can only be 2,5,9.

In column 2, after the elimination I mentioned above, 4 only shows up in r1c2 and r2c2. So 4 can be eliminated from the other cells in box 1 (r1c1-r3c3). Which means, eventhough r3c3 had 4 and 9, 4 can be eliminated, leaving only the 9.
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squirrel



Joined: 24 Aug 2005
Posts: 3

PostPosted: Wed Aug 24, 2005 9:31 pm    Post subject: Reply with quote

There are hidden pairs {2,5} at r5c7 and r5c9. That means a 4 must appear in either r5c1 or r5c3 and thus cannot appear in r4c2 or r6c2.

Also, since there's already a 4 in both r8 and c5, there must be a 4 in either r9c4 or r9c6.

We have now eliminated 4 as a candidate from every row in c2 except for r1c2 and r2c2. There must be a 4 in one of these, and thus 4 cannot appear in r3c3.
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squirrel



Joined: 24 Aug 2005
Posts: 3

PostPosted: Wed Aug 24, 2005 9:32 pm    Post subject: Reply with quote

Note to self: Next time, type faster. Very Happy
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PostPosted: Thu Aug 25, 2005 6:47 am    Post subject: Reply with quote

Thanks for your efforts. This time I found the solution (4 only possible in c2 in box 1) myself, but unfortunately only after I left my computer access, so I could not stop you!
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