Welcome back to this short series showing how to solve sudoku puzzles from beginning to end.
At this point in the puzzle, we are starting to see several nearly complete houses and so we can either complete them or use them to “lock down” candidates that we can then eliminate from other boxes.
In the grid following on from the last post, you can see this at work in cells 8,8 and 9,8. There are only two values that are still left in that box and so they can not exist in the rest of the whole of column 8.
The only values that can go in box 9, are 6 and 9 and this means that 1 and 7 are the only other possible candidates for the rest of the column. Looking in box 2, you can see a 1 in row 1 so column 8 can only have a 7 in the top right box. That means that the 1 must go in box 6. Following on from that, you should see how we can chase the 7’s all round the grid, filling in as we go.
The next step is to spot the naked single in box 3 in cell 2,7….only 3 is left in that column. Then we can fill in the last two values for box 3, using that 1 in box 2 again as a constraint.
You can see that I have also filled some other values where their houses were almost complete. As you get close to the end of a puzzle, this will happen more and more often.
I have highlighted two more cells in box 6, as I think this is the first really clear example of a naked pair in this puzzle. If you look along row 6, you should see that the only possible values are 1,3,5 and 8. We already have 1 and 8 in both box 5 and 6, so these two boxes can only possibly be 3 or 5 in those cells. You could look at it as if the 1 and 8 have been forced out into box 4 for this row.
This is great because it gives us a second naked pair in box 4. That means that none of the other cells in box 4 can have a 1 or 8 in them and this cuts down our choice for the last 3 cells to 3,5, or 6. Your homework for this post is to go to work on those three candidates, to find one of them that fits perfectly (with the constraints around) and to use that to finish the row and any other cells in box 4 that you can manage.
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