Sudoku Solving Techniques

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Technique 8 :

Guessing 

To find a cell with just a couple of candidates, and to pick one of the candidates. If you work through from that point, and it turns out to be solvable and a valid grid, then great, you guessed right!  If your guess was wrong, maybe it would show itself up within just a couple more moves, maybe it would take right until the very end before the last number was incompatible... Do you feel lucky? Some people will try just a few moves, before deleting them and trying another for speed.

TIPS 

1. Try picking a value for a cell which looks like it will immediately force lots of other values. That way you can be sure that you'll get good reward for your effort if it does turn out to have a contradiction.
2. Try picking three or four values and just working out a short chain from each. If you don't find one quickly, then move on and try another. There's often a very short chain nearby, and its worth a quick search to find it before you start the much longer and more meticulous searches.
3. An overlay (tracing paper or a computer equivalent) really helps you to try out various chains without you making a complete mess of your original!.

Sometimes you'll see the contradiction (error) within just a couple of placements, sometimes you'll get nearly to the end.

There is great saying,

" A positive isn't proof, but a negative is a disproof! "

(click here for rules)