I know the concept of pairing and removed pairs but what other concepts should I use to solve this?
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$\begingroup$ You could try guessing and check if it works in your mind, like when there are two possibilities, like in the bottom right square. $\endgroup$ – Duck Oct 7 '19 at 16:23
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There is only one place to put
a 6 in column 1
followed by the
6 in row 6
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$\begingroup$ @030 Is it not obvious? There is only one cell in the first column that still contains a small 6. $\endgroup$ – Jaap Scherphuis Oct 14 '19 at 17:57