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what's the technique to break this? Thanks.

Hint : there are only 3 solutions for the digit 5.

Code: Select all

*--------------------------------------------------------------*
| 6    *578   378    | 2     1     4      |*35    9    *357    |
| 57    1     4      | 79    5789  3      | 6     578   2      |
| 357   9     2      | 6     578   58     | 34-5  4578  1      |
|--------------------+--------------------+--------------------|
| 4789  47    5      | 1     3478  6      | 349   2     347    |
| 4789  2     6      | 37    3478  89     |*3459  1    *3457   |
| 479   3     1      | 5     247   29     | 8     47    6      |
|--------------------+--------------------+--------------------|
| 1     48-5  38     | 39    359   7      | 2     6     48-5   |
| 2    *58    9      | 4     6     1      | 7     3    *58     |
| 3457  6     37     | 8     235   25     | 1     45    9      |
*--------------------------------------------------------------*

There is a Swordfish in 5's (the cells marked *) in r158 / c279 which eliminates the 5's in r3c7, r7c2 and r7c9.

This => r7c5 = 5 and the puzzle then solves easily.

Just prior to this you can remove the 8's from r2c1 and r3c1 because either r1c2 or r1c3 must be 8. You can also remove the 8 from r4c2 because either r4c1 or r5c1 must then be 8.

Leren