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Hessah, I would LOVE to say that the answer (for everything, really) can be directly obtained by logical reasoning. However, the reality is that we sometimes have to resort to trials and errors. We can, though, limit the number of trials. In all sudokus of this level, I always manage to nail it down to 2 possible pathways. I try one and if it doesn't work, the other one is definitely the road to victory. The trick is to choose which 2 pathways. Usually, it's two boxes in one 3x3 square that one number can go in and if either box is filled, it must also reveal the others in the puzzle, until you encounter a contradiction where the same number turns up twice in a row, column or a 3x3 square (when you choose the wrong path) or complete the puzzle (if you happen to choose the correct path in the first place). If you make a bad choice, the box that is filled in won't lead to many others and you end up stuck again.
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Primum non nocere
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