CageDokuKiller Sudoku solving tips and techniques
Seven techniques, in the order you should try them. Every puzzle in CageDoku is solvable with these.
1. Unique cage combinations
Some sums have only one possible set of digits: two cells 3 = {1,2}, 4 = {1,3}, 16 = {7,9}, 17 = {8,9}; three cells 6 = {1,2,3}, 7 = {1,2,4}, 23 = {6,8,9}, 24 = {7,8,9}. Write the candidates in at once. See the full chart.
2. The rule of 45 (innies and outies)
A row, column or box sums to 45. If a region is covered by cages except one cell (an innie), that cell equals 45 minus the sums of the full cages. If a cage pokes one cell out of the region (an outie), the outie equals the cage total minus the part inside. Extend the idea to two or three adjacent rows or boxes.
3. Combination elimination
A 3-cell cage with sum 8 can only be {1,2,5} or {1,3,4}. Candidates 6, 7, 8 and 9 are impossible in it, and the digit 1 is certain somewhere in the cage because it appears in both combinations. Once one cell of the cage is known to be 5, only {1,2,5} remains.
4. Min/max cell values
A 2-cell cage with sum 11 cannot contain 1 (it would need a 10). Likewise sum 4 cannot contain 4. The extremes of each cage trim candidates quickly.
5. Hidden and naked singles
These classic Sudoku techniques still apply: a digit that fits in only one cell of a row, column or box, or a cell with only one candidate left.
6. Locked candidates, pairs and triples
When a digit in a box can only go in one row, it cannot appear elsewhere in that row. Naked and hidden pairs or triples remove candidates from the rest of their unit. Cage logic often creates these patterns.
7. Chains and fish (Extreme)
X-Wing, swordfish and simple chains appear in the hardest levels. If you need them, CageDoku's hint explains each pattern with the cells highlighted.
Frequently asked questions
How do I start a Killer Sudoku?
Scan for cages with a unique combination (2 cells: 3, 4, 16, 17; 3 cells: 6, 7, 23, 24), then apply the rule of 45 to rows, columns and boxes to find innies and outies.
Do I need to guess in Killer Sudoku?
No. A well-made Killer Sudoku has a unique solution reachable by logic alone. CageDoku rates every puzzle by the hardest human technique required.