Code: Select all
. . 5|. . .|. 6 9
7 3 4|8 6 9|. . .
6 9 .|. . .|. 4 3
-----+-----+-----
. 4 .|6 . 5|. 9 .
5 . .|2 . 4|6 . .
. 1 6|. 8 .|5 . 4
-----+-----+-----
. 5 .|. 2 6|. 1 .
4 6 .|. . .|3 . .
. . .|3 . .|. . 6
After the initial steps you come up with the situation below.
Code: Select all
.------------------.------------------.------------------.
| 128 28 5 | 147 347 1237 | 78 6 9 |
| 7 3 4 | 8 6 9 | 12 25A 125 |
| 6 9 128 | 157 57 127 | 78 4 3 |
:------------------+------------------+------------------:
| 238 4 2378 | 6 137 5 | 12B 9 1278B|
| 5 78 3789 | 2 1379 4 | 6 378 178B |
| 239 1 6 | 79 8 37 | 5 -237 4 |
:------------------+------------------+------------------:
| 389 5 3789 | 479 2 6 | 49 1 78AB |
| 4 6 12789| 1579 579 178 | 3 2578A 25-7-8|
| 1289 278 12789| 3 457 178 | 49 2578A 6 |
'------------------'------------------'------------------'
Take the 4 squares marked with an A. The squares in R89C8 can't both be 7 or 8 cause this would interfere with R7C9 and can't be both 2 or 5 because this would interfere with R2C8. So R89C8 contains either a 2 or a 5 and either a 7 or an 8. This creates a "naked pair" 7-8 in box 9 with R7C9 which eliminates 7 and 8 from the remaining squares of box 9. It also creates a "naked pair" 2-5 with R2C8 for column 8 which eliminates 2 and 5 from the remaining squares in column 8.
So we can eliminate the 2 from R6C8 and the 7 and 8 from R8C9.
The same goes for the squares marked with B for 1 and 2 in box 6 and 7 and 8 in column 9. This makes the same eliminations.
Para