SUDOKU #9  
 
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.  
3  1  7  8  
1  9  
4  9  6  
6  9  8  
7  6  
5  4  1  
1  5  9  
2  6  
8  3  5  2  
This is a Evil difficulty puzzle (harder than Hard), 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  3  1  7  8  
B  1  9  
C  4  9  6  
D  6  9  8  
E  7  6  
F  5  4  1  
G  1  5  9  
H  2  6  
I  8  3  5  2  
J  K  L  M  N  O  P  Q  R  
There is a 9 in AL (because of CM and BQ). There is a 5 in HL (because of GO and IQ). There is a 6 in FO (because of DJ, EQ, and HM) and another 6 in AN (because of CR and HM).  
A  3  1  9  6  7  8  
B  1  9  
C  4  9  6  
D  6  9  8  
E  7  6  
F  5  4  6  1  
G  1  5  9  
H  2  5  6  
I  8  3  5  2  
J  K  L  M  N  O  P  Q  R  
There has to be a 4 in either AM or BM (because of CJ and FN). There has to be a 4 or 9 in HO and IO (because of the 4 in AMBM and FN, and because of the 9s in CM and DN). There's an 8 and 9 in EJ and FK (because of AL, DN, DR, and IL).  
A  3  1  9  6  7  8  
B  1  9  
C  4  9  6  
D  6  9  8  
E  89  7  6  
F  5  89  4  6  1  
G  1  5  9  
H  2  5  6  49  
I  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
There's a 2 in BJ (because of HK). So there is a 5 or 8 in BK and CK (because of HL and IL). That means that there is an 8 in EJ and a 9 in FK. And there's a 6 and 7 in BL and CL, which means there is a 6 in BL (because of CR) and a 7 in CL. The missing numbers in column J are the 7 and 9.  
A  3  1  9  6  7  8  
B  2  58  6  1  9  
C  4  58  7  9  6  
D  6  9  8  
E  8  7  6  
F  5  9  4  6  1  
G  1  5  9  
H  79  2  5  6  49  
I  79  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
There is a 9 in EP (because of FK, DN, and GR). There's an 8 in FM (because of DR and EJ) and another one in CO (because of FM). That means there is an 8 in BK and a 5 in CK. There's a 2 and a 3 in DO and EO. There's a 7 in DM (because of AO and EK).  
A  3  1  9  6  7  8  
B  2  8  6  1  9  
C  4  5  7  9  8  6  
D  6  7  9  23  8  
E  8  7  23  9  6  
F  5  9  8  4  6  1  
G  1  5  9  
H  79  2  5  6  49  
I  79  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
There's a 1 and a 5 in EM and EN. Therefore, there's a 5 in DP (because of EM/EN, FJ, and IQ), and there's a 1 in DL (because of EM/EN, FR, and AK).  
A  3  1  9  6  7  8  
B  2  8  6  1  9  
C  4  5  7  9  8  6  
D  6  1  7  9  23  5  8  
E  8  7  15  15  23  9  6  
F  5  9  8  4  6  1  
G  1  5  9  
H  79  2  5  6  49  
I  79  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
The numbers missing from box GMIO are 1, 2, 7, 8. There is a 1 in GJ and a 7 and 8 in column M, so there has to be a 2 in GM. That means that there has to be a 2 in CN (because of GM and DO/EO) and another 2 in AQ (because of IR, BJ, and CN).  
A  3  1  9  6  7  8  2  
B  2  8  6  1  9  
C  4  5  7  9  2  8  6  
D  6  1  7  9  23  5  8  
E  8  7  15  15  23  9  6  
F  5  9  8  4  6  1  
G  1  2  5  9  
H  79  2  5  6  49  
I  79  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
There's a 3 in BN (because of AJ and IM). That means that there is a 4 and a 5 in AM and BM  and AR, which means EM has to be a 1 and EN has to be a 5.  
A  3  1  9  45  6  7  8  2  45  
B  2  8  6  45  3  1  9  
C  4  5  7  9  2  8  6  
D  6  1  7  9  23  5  8  
E  8  7  1  5  23  9  6  
F  5  9  8  4  6  1  
G  1  2  5  9  
H  79  2  5  6  49  
I  79  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
The numbers missing from row D are the 2, 3, and 4, but there is a 2 in HK and AQ, so the 2 has to be in DO  which means the 3 is in EO. There's another 2 in FP (because of IR and AQ). So there is a 2 in EL and a 4 in ER. That means there is a 4 in DK, a 3 in DQ, a 4 in ER, a 7 in FQ, a 3 in FL, and a 4 in GL.  
A  3  1  9  45  6  7  8  2  45  
B  2  8  6  45  3  1  9  
C  4  5  7  9  2  8  6  
D  6  4  1  7  9  2  5  3  8  
E  8  7  2  1  5  3  9  6  4  
F  5  9  3  8  4  6  2  7  1  
G  1  4  2  5  9  
H  79  2  5  6  49  
I  79  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
Wow! There is a 3 in IM, so a 3 has to be in GK. That means a 6 is in IK, which means a 6 is in GP (because of HM and EQ).  
A  3  1  9  45  6  7  8  2  45  
B  2  8  6  45  3  1  9  
C  4  5  7  9  2  8  6  
D  6  4  1  7  9  2  5  3  8  
E  8  7  2  1  5  3  9  6  4  
F  5  9  3  8  4  6  2  7  1  
G  1  3  4  2  5  6  9  
H  79  2  5  6  49  
I  79  6  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
There's a 4 in ER, so there has to be a 5 in AR, a 4 in AM, and a 5 in BM. There is a 5 in BP (because of CJ and ER), so there is a 7 in BR and a 3 in HR.  
A  3  1  9  4  6  7  8  2  5  
B  2  8  6  5  3  1  4  9  7  
C  4  5  7  9  2  8  6  
D  6  4  1  7  9  2  5  3  8  
E  8  7  2  1  5  3  9  6  4  
F  5  9  3  8  4  6  2  7  1  
G  1  3  4  2  5  6  9  
H  79  2  5  6  49  3  
I  79  6  8  3  49  5  2  
J  K  L  M  N  O  P  Q  R  
There is a 3 in CP (because of DQ) and a 1 in CQ. There's a 7 in GN (because of FQ), an 8 in GQ, and a 4 in HQ. That means there is a 4 in IO, a 9 in HO, a 7 in HJ, and a 9 in IJ.  
A  3  1  9  4  6  7  8  2  5  
B  2  8  6  5  3  1  4  9  7  
C  4  5  7  9  2  8  3  1  6  
D  6  4  1  7  9  2  5  3  8  
E  8  7  2  1  5  3  9  6  4  
F  5  9  3  8  4  6  2  7  1  
G  1  3  4  2  7  5  6  8  9  
H  7  2  5  6  9  4  3  
I  9  6  8  3  4  5  2  
J  K  L  M  N  O  P  Q  R  
That means there is a 7 in IP, a 1 in HP, a 1 in IN, and an 8 in HN.  
A  3  1  9  4  6  7  8  2  5  
B  2  8  6  5  3  1  4  9  7  
C  4  5  7  9  2  8  3  1  6  
D  6  4  1  7  9  2  5  3  8  
E  8  7  2  1  5  3  9  6  4  
F  5  9  3  8  4  6  2  7  1  
G  1  3  4  2  7  5  6  8  9  
H  7  2  5  6  8  9  1  4  3  
I  9  6  8  3  1  4  7  5  2  
J  K  L  M  N  O  P  Q  R  
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