Killer Sudoku cage combinations worth memorising
Killer Sudoku looks impenetrable until you notice that some cages can only be filled one way. Those cages are the handholds, and once you know them an empty grid stops being intimidating.
A cage never repeats a digit, which is what makes so many totals unique.
Play Killer SudokuTwo square cages with one answer
- 3 is 1 and 2. 4 is 1 and 3. 5 is 1 and 4, or 2 and 3, so it is not unique.
- 16 is 7 and 9. 17 is 8 and 9.
- In short: 3, 4, 16 and 17 are gifts. Everything between has options.
Three square cages with one answer
- 6 is 1, 2, 3. 7 is 1, 2, 4.
- 23 is 6, 8, 9. 24 is 7, 8, 9.
- 8 and 22 each have two combinations, which is still narrow enough to be useful.
Four square cages with one answer
- 10 is 1, 2, 3, 4. 11 is 1, 2, 3, 5.
- 29 is 5, 7, 8, 9. 30 is 6, 7, 8, 9.
The rule of 45
Every row, column and box contains 1 to 9, so each totals 45. If cages cover a unit exactly, their totals add to 45. If they cover it except for one square, that square is 45 minus the totals.
This is the single most useful technique in the game, and it is the one beginners never think to try.
Play Killer SudokuHow to use them in order
- Mark the unique cages first, as candidates rather than answers.
- Apply the rule of 45 to any row, column or box that cages nearly cover.
- Switch to ordinary Sudoku scanning as soon as a few digits are placed.
- Go back to the sums when the scanning stalls. The two keep feeding each other.
Learn the two square list and the rule of 45 and you have most of what a hard Killer asks for. The rest is the Sudoku you already know.
Play Killer Sudoku