- Code: Select all
*-----------*
|...|..1|.45|
|.3.|...|...|
|8..|3..|..6|
|---+---+---|
|7..|.9.|1..|
|.5.|..2|..9|
|...|7..|...|
|---+---+---|
|5.6|.2.|...|
|.2.|.1.|...|
|..9|.8.|.54|
*-----------*
This is where I think you are, when you make your error.
- Code: Select all
*---------------------------------------------*
| 29 69 27 | 8 67 1 | 3 4 5 |
| 46 3 457 | 2 4567 469 | 8 19A 17B |
| 8 14 1457 | 3 457 b49 |a79 2 6 |
|--------------+---------------+--------------|
| 7 46 234 | 45 9 4568 | 1 368 238 |
| 36 5 8 | 1 36 2 | 4 7 9 |
| 29 1469 1234 | 7 346 468 | 5 368 238 |
|--------------+---------------+--------------|
| 5 8 6 | 49 2 c47 | 9-7C 13 13 |
| 34 2 34 | 59 1 57 | 6 89 78 |
| 1 7 9 | 6 8 3 | 2 5 4 |
*---------------------------------------------*
There is an XY Wing in the cells marked abc that removes 7 from r7c7, which I think you would call G7.
In words, this works as follows : If cell a is not 7 it's 9, so cell b is not 9, so it's 4, so cell c is not 4, so it's 7. You can reverse the argument, start at cell c, assume it's not 7 and conclude that cell a is 7.
The result of all of this is that at least one of cells a and c must be 7. So r7c7, which can see cells a and c, can't be 7.
I've also marked the cells you call H2, I2, G7 as A, B and C. There is no XY wing in those three cells
because cells A and B don't see cell C, so the argumentation I used above breaks down.
Some links to good teaching sites that explain how XY Wings work are
here and
here.
Leren