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Hidden Singles

The hidden singles technique is shared with regular Sudoku puzzles, and is discussed in that context here. The technique is identical in Killer puzzles. Simply stated if there is only one place in a group (a row, column or box) where a digit can go, then it must go there. This case is most easily spotted using notes (which show which digits could go where). Here is an example:

In this case, the cell highlighted in red must be a "2", because that digit has been eliminated from everywhere else in the column highlighted in orange. (In this example and the examples that follow, the notes are only displayed in the highlighted cells, to avoid confusion).

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