SUDOKU #7  
 

I'm going to repeat the info at the top of Sudoku #1, just in case you haven't seen that one yet.
 
I've been doing Sudoku for a while. There are a lot of people like me who have figured out some tricks or techniques that help them complete the puzzles. This page is not really for them. I met a woman recently who was just starting to try to do Sudoku. This page is for people like her. I'm not saying that I know every single trick, but I know enough to complete most puzzles.
 
The basic rules of Sudoku are that there is only one of each number in each row, column, and 9 square grid. There isn't any mathematics or arithmetic. If you know how to count from 1 to 9, then you can do these puzzles. All of the techniques are based on this rule.
 
Here is a sample Sudoku puzzle.
 
      7 8       3 1    
    4               5  
    2     7   8     4  
      4     1     3    
    1     5   3     6  
      5     6     9    
    9     8   2     7  
    3               9  
      6 5       8 2    
 
This is a Medium difficulty puzzle, and it will allow me to show some examples of a few of the tricks that I've uncovered. I've added the letters below to the rows and columns in order to talk about each of the 81 squares. As a convention, I will talk about squares and boxes. There are 81 squares, but there are 9 boxes of 9 squares.
     
  A   7 8       3 1    
  B 4               5  
  C 2     7   8     4  
  D   4     1     3    
  E 1     5   3     6  
  F   5     6     9    
  G 9     8   2     7  
  H 3               9  
  I   6 5       8 2    
  J K L M N O P Q R  
                         
Notice that there is a 7 and 8 in box AJ-CL in row A, and another pair in box AM-CO in row C, so there has to be a 7 and 8 in BP and BQ. There is an 8 in IP, so the 8 is in BQ and the 7 is in BP. That means that there is a 7 in EQ (because of GR and BP).
         
  A   7 8       3 1      
  B 4           7 8 5    
  C 2     7   8     4    
  D   4     1     3      
  E 1     5   3   7 6    
  F   5     6     9      
  G 9     8   2     7    
  H 3               9    
  I   6 5       8 2      
    J K L M N O P Q R    
                             
There is a 5 in AJ (because of FK and IL), another 5 in CN (because of AJ and BR), a 5 in HO (because of EM, CN, and IL), a 5 in DP (because of BR, FK, and EM), and the final 5 is in GQ (because of BR, DP, and IL).
             
  A 5 7 8       3 1      
  B 4           7 8 5    
  C 2     7 5 8     4    
  D   4     1   5 3      
  E 1     5   3   7 6    
  F   5     6     9      
  G 9     8   2   5 7    
  H 3         5     9    
  I   6 5       8 2      
    J K L M N O P Q R    
                         
There is a 3 in IR (because of AP and DQ), a 3 in GN (because of IR and HJ), a 3 in BM (because of GN and EO), a 3 in FL (because of HJ, DQ, and EO), and a 3 in CK (because of AP, BM, and FL).
         
  A 5 7 8       3 1      
  B 4     3     7 8 5    
  C 2 3   7 5 8     4    
  D   4     1   5 3      
  E 1     5   3   7 6    
  F   5 3   6     9      
  G 9     8 3 2   5 7    
  H 3         5     9    
  I   6 5       8 2 3    
    J K L M N O P Q R    
                         
There is a 1 in BO (because of AQ and DN), a 1 in FR (because of AQ, DN, and EJ), a 1 in CL (because of AQ and BO). There is a 1 in IM (because of EJ, DN, and BO).
         
  A 5 7 8       3 1      
  B 4     3   1 7 8 5    
  C 2 3 1 7 5 8     4    
  D   4     1   5 3      
  E 1     5   3   7 6    
  F   5 3   6     9 1    
  G 9     8 3 2   5 7    
  H 3         5     9    
  I   6 5 1     8 2 3    
    J K L M N O P Q R    
                         
The numbers missing from box AJ-CL are the 6 and 9, so there is a 2 in BN -- which means there is a 2 in AR (because of CJ and BN).
         
  A 5 7 8       3 1 2    
  B 4     3 2 1 7 8 5    
  C 2 3 1 7 5 8     4    
  D   4     1   5 3      
  E 1     5   3   7 6    
  F   5 3   6     9 1    
  G 9     8 3 2   5 7    
  H 3         5     9    
  I   6 5 1     8 2 3    
    J K L M N O P Q R    
                         
The numbers missing from box AP-CR are 6 and 9, but there is a 9 in FQ, so the 9 is in CP and the 6 is in CQ. That means there is a 4 in HQ.
         
  A 5 7 8       3 1 2    
  B 4     3 2 1 7 8 5    
  C 2 3 1 7 5 8 9 6 4    
  D   4     1   5 3      
  E 1     5   3   7 6    
  F   5 3   6     9 1    
  G 9     8 3 2   5 7    
  H 3         5   4 9    
  I   6 5 1     8 2 3    
    J K L M N O P Q R    
                         
The numbers missing from box GP-IR are the 1 and 6, which means the numbers missing from EP and FP are the 2 and 4, which means that the number in DR is an 8. There is an 8 in HK (because of IP, GM, and AL), an 8 in FJ (because of AL, HK, and DR), and there is an 8 in EN (because of FJ and DR).
         
  A 5 7 8       3 1 2    
  B 4     3 2 1 7 8 5    
  C 2 3 1 7 5 8 9 6 4    
  D   4     1   5 3 8    
  E 1     5 8 3   7 6    
  F 8 5 3   6     9 1    
  G 9     8 3 2   5 7    
  H 3 8       5   4 9    
  I   6 5 1     8 2 3    
    J K L M N O P Q R    
                         
There is a 1 in GK (because of AL and EJ) and another 1 in HP (because of GK).
         
  A 5 7 8       3 1 2    
  B 4     3 2 1 7 8 5    
  C 2 3 1 7 5 8 9 6 4    
  D   4     1   5 3 8    
  E 1     5 8 3   7 6    
  F 8 5 3   6     9 1    
  G 9 1   8 3 2   5 7    
  H 3 8       5 1 4 9    
  I   6 5 1     8 2 3    
    J K L M N O P Q R    
                         
There is a 6 in BL (because of IK) and a 9 in BK. There is another 6 in DJ (because of BL and IK). There is a 2 in EK, so there is a 2 in FP and a 4 in EP, and there is a 2 in HL (because of GO and IQ). The final 2 is in DM (because of FP and GO). There is a 7 in IJ, a 4 in GL, and a 6 in GP.
         
  A 5 7 8       3 1 2    
  B 4 9 6 3 2 1 7 8 5    
  C 2 3 1 7 5 8 9 6 4    
  D 6 4   2 1   5 3 8    
  E 1 2   5 8 3 4 7 6    
  F 8 5 3   6   2 9 1    
  G 9 1 4 8 3 2 6 5 7    
  H 3 8 2     5 1 4 9    
  I 7 6 5 1     8 2 3    
    J K L M N O P Q R    
                         
There is a 9 in EL, a 7 in DL, and a 9 in DO. There is a 9 in IN (because of HR and DO) and a 9 in AM (because of IN and DO).
         
  A 5 7 8 9     3 1 2    
  B 4 9 6 3 2 1 7 8 5    
  C 2 3 1 7 5 8 9 6 4    
  D 6 4 7 2 1 9 5 3 8    
  E 1 2 9 5 8 3 4 7 6    
  F 8 5 3   6   2 9 1    
  G 9 1 4 8 3 2 6 5 7    
  H 3 8 2     5 1 4 9    
  I 7 6 5 1 9   8 2 3    
    J K L M N O P Q R    
                         
There is a 6 in AO, a 4 in AN, a 7 in HN.
         
  A 5 7 8 9 4 6 3 1 2    
  B 4 9 6 3 2 1 7 8 5    
  C 2 3 1 7 5 8 9 6 4    
  D 6 4 7 2 1 9 5 3 8    
  E 1 2 9 5 8 3 4 7 6    
  F 8 5 3   6   2 9 1    
  G 9 1 4 8 3 2 6 5 7    
  H 3 8 2   7 5 1 4 9    
  I 7 6 5 1 9   8 2 3    
    J K L M N O P Q R    
                         
There is a 7 in FO (because of CM), a 4 in FM, a 4 in IO, and a 6 in HM.
         
  A 5 7 8 9 4 6 3 1 2    
  B 4 9 6 3 2 1 7 8 5    
  C 2 3 1 7 5 8 9 6 4    
  D 6 4 7 2 1 9 5 3 8    
  E 1 2 9 5 8 3 4 7 6    
  F 8 5 3 4 6 7 2 9 1    
  G 9 1 4 8 3 2 6 5 7    
  H 3 8 2 6 7 5 1 4 9    
  I 7 6 5 1 9 4 8 2 3    
    J K L M N O P Q R    
 
 
HOME