Hard Killer Sudoku
Hard Killer Sudoku puzzles are perfect for players who already feel confident enough in their solving skills and are ready for an upgrade that also challenges their arithmetic skills. Unlike previous difficulty levels that can still be considered generally easier than their equivalents on classic Sudoku, these puzzles already do justice to the Killer title of this variant. With fewer but larger cages with more complex sums, these puzzles require the players’ full attention and use of their brain cells.
The 45-rule to solve hard Killer Sudoku puzzles
Solving hard puzzles of online Killer Sudoku is almost impossible if the players are not familiar with the 45-rule and know how to employ it and take the most advantage of it.
This rule applies only to 9 x 9 grids. Since there cannot be any digit repetition per column, group, or row, the sum of the numbers in each of these is always 45 (1+2+3+4+5+6+7+8+9 = 45). In practice, this means you can use the known numbers in a row, column, or group and their sum (in the case of cages) to infer the missing values by subtracting them from 45.
This criterion can also be used to make sure that the totals of the numbers in cages that intersect line up correctly, which can help you find and fix mistakes. In other words, you can cross-check your progress, make smart predictions, and methodically reduce the options for each cell by consistently using the 45 rule.
