by Rick Yakubisin
Using Math in Solving Sudoku
Any row, column or grid.
Pick a candidate and count how many the chosen candidate is in the row, column or grid.
Count the cells that are filled in for the row, column, or grid. Add the two counts together. Subtract from nine.
If the number of integers in the remaining cells is less than or equal to the calculation then there is a hidden pair, triplet, or quad. Of course, if they are not hidden it’s not of value.
1) If you look at row candidate 6 in row 6 you get a count of 3
2) There are 2 completed cells(3,9) in row 6 for a count of 2
3) add the 2 counts and you get 5
4) subtract 5 from 9 for the result of 4. So there are 4 cells in row 6 that do not contain 6 and or not complete
5) count the integers in the remaining 4 cells. In this case the 4 cells contains the integers 2,4,7 and 8 for a count of 4
6) since there are 4 cells left that only have 4 candidates you know you can uncover some hidden stuff.
7) in this example you can remove the 2, 4, 7 and 8 from all cells that have the candidate 6 in it
Seems complex but it is pretty easy to spot after you do it a few times