Killer Sudoku looks intimidating — a Sudoku grid covered in colored cages with small numbers in the corners. But once you understand how the cages work, the puzzle opens up in ways regular Sudoku cannot.
Killer Sudoku has all the standard Sudoku rules: digits 1–9 appear exactly once in each row, column, and 3×3 box. But there are also cages — groups of 2 to 5 connected cells, outlined in color. The number in the corner of each cage is the sum of all digits in that cage. No digit may repeat within a cage.
The most powerful starting point is a 2-cell cage with a small sum. A cage summing to 3 must contain 1 and 2 — there is no other way. A cage summing to 4 must contain 1 and 3. These "forced pair" cages immediately give you two digits in specific cells.
Every row, column, and 3×3 box contains the digits 1 through 9, which sum to exactly 45. This is a powerful constraint. If all but one cage in a row is complete, the remaining cage's sum within that row is 45 minus the total of the other cages.
Even when you cannot determine exact values, cages eliminate candidates. If a cage in a column contains cells in only one row within a 3×3 box, then the remaining cells in that row within the box cannot contain any digits already placed in the cage.
This is often forgotten by beginners. A cage summing to 10 with 3 cells could be {1,2,7}, {1,3,6}, {1,4,5}, or {2,3,5} — but never {1,4,5} and {2,3,5} simultaneously. Once you eliminate some combinations via row/column constraints, the remaining options narrow quickly.
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