Mal
Posts: 143
Joined: 6/26/2002
From: United Kingdom
Status: offline
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Hi there
The first thing I spotted with this puzzle was an X-wing of the 1s in Rows 1 and 9.
Have a look at the 1s in row 1 (box 1, cell 1 and box 2 cell 3. Compare that with the 1s in Row 9 box 7, cell 7 and box 8, cell 9.
An X-wing is where a candidate has only 2 locations in a row and the same candidate has only 2 locations in a different row, but occupying the same columns. In this puzzle columns 1 & 6. All other instances of the 1s can be eliminated from those columns - column 1, box 7, cell 4 and column 6, box 2, cell 9. The same logic can be applied comparing columns and then eliminating from the rows. I don't know why this technique is called an X-wing, because really it forms a square or rectangle!
In box 2 you will then have a 7,9 pair in cells 1 and 9. You can then eliminate the 7s and 9s from the other cells in that box. You will then have a 5 in cell 7. See how you go from there.
There are lots of strategies you can find out about on the web. Start off with basic strategies, e.g hidden pairs, hidden triples. The X-wing is often classed as a tough strategy, but I find them easier to spot than hidden triples! There are a lot of advanced strategies but personally, in all the years I've been doing Sudoku puzzles and variants, I've never had to use a strategy higher than an X-wing.
Good luck.
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