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Medium Puzzle #49 - One Possible Solution Approach…
NB:  There is only one final solution to this Sudoku puzzle…
A B C D E F G H I
1                  
2 AA BB CC Reference to 3x3 Blocks AA, BB & CC
3                  
4                  
5 DD EE FF Reference to 3x3 Blocks DD, EE & FF
6                  
7                  
8 GG HH II Reference to 3x3 Blocks GG, HH & II
9                  
A B C D E F G H I
1 4       7       2 Medium Puzzle #49 - November 18, 2005
2   3     9     4   Original puzzle as published © joe-ks.com
3     6   8   3    
4 1     5 2 7     9
5 8 7 9 1 6 3 4 2 5
6 5     8 4 9     3
7     4   1   8    
8   2     3     7  
9 6       5       1
A B C D E F G H I
1 4     3 7       2 Step 1: #3 is in Rows 2 & 3: #3 MUST go in Cell D1
2   3     9     4        (#3 in Column F @ F5 excludes use of Cell F1)
3     6   8   3    
4 1     5 2 7   8 9 Step 2: #8 is in Rows 5 & 6: #8 MUST go in Cell H4
5 8 7 9 1 6 3 4 2 5      (#8 in Column G @ G7 excludes use of Cell G4)
6 5     8 4 9     3
7     4   1   8     Step 3: #1 is in Rows 7 & 9: #1 MUST go in Cell C8
8   2 1   3     7        (#1 in Column A @ A4 excludes use of Cell A8)
9 6       5       1
A B C D E F G H I
1 4     3 7       2 Step 4: #4 is in Columns A & C: #4 MUST go in Cell B4
2   3     9     4        (#4 in Row 6 @ E6 excludes use of Cell B6)
3     6   8   3    
4 1 4   5 2 7   8 9 Step 5: #2 is in Columns H & I: #2 MUST go in Cell G9
5 8 7 9 1 6 3 4 2 5       (#2 in Row 8 @ B8 excludes use of Cell G8)
6 5     8 4 9     3
7     4   1   8    
8   2 1   3     7  
9 6       5   2   1
A B C D E F G H I
1 4     3 7       2 Step 6: #6 is in Columns A & C: #6 MUST go in Cell B6...
2   3     9     4  
3     6   8   3     Step 7: #6 is now in Rows 5 & 6: #6 MUST go in Cell G4...
4 1 4 3 5 2 7 6 8 9
5 8 7 9 1 6 3 4 2 5 Step 8: Complete Row 4: #3 MUST go in Cell C4...
6 5 6   8 4 9     3
7     4   1   8    
8   2 1   3     7  
9 6       5   2   1
A B C D E F G H I
1 4     3 7       2 Step 9: Complete Block DD: #2 MUST go in Cell C6...
2   3     9     4  
3     6   8   3     Step 10: Complete Block FF: need #s 1 & 7…
4 1 4 3 5 2 7 6 8 9      #7 can't go in Cell H6 (cf #7 in Column H @ H8), so
5 8 7 9 1 6 3 4 2 5      #7 MUST go in Cell G6, & therefore #1 MUST go in Cell H6
6 5 6 2 8 4 9 7 1 3
7     4   1   8    
8   2 1   3     7  
9 6       5   2   1
A B C D E F G H I
1 4     3 7       2 Step 11: #4 is in Columns G& H: #4 MUST go in Cell I8
2   3     9     4        (#4 in Row 7 @ C7 excludes use of Cell I7)
3     6   8   3    
4 1 4 3 5 2 7 6 8 9 Step 12: #3 is in Columns B & C: #3 MUST go in Cell A7
5 8 7 9 1 6 3 4 2 5      (#3 in Row 8 @ E8 excludes use of Cell A8)
6 5 6 2 8 4 9 7 1 3
7 3   4   1   8     Step 13: #3 is now in Rows 7 & 8: #3 MUST go in Cell H9...
8   2 1   3     7 4
9 6       5   2 3 1
A B C D E F G H I
1 4     3 7       2 Step 14: #8 is in Columns G & H: #8 MUST go in Cell I2
2   3     9     4 8     (#8 in Row 3 @ E3 excludes use of Cell I3)
3     6   8   3    
4 1 4 3 5 2 7 6 8 9
5 8 7 9 1 6 3 4 2 5
6 5 6 2 8 4 9 7 1 3
7 3   4   1   8    
8   2 1   3     7 4
9 6       5   2 3 1
A B C D E F G H I
1 4     3 7       2 Step 15: Complete Column I: need #s 6 & 7…
2   3     9     4 8      #6 can't go in Cell I3 (cf #6 in Row 3 @ C3), so
3     6   8   3   7      #6 MUST go in Cell I7, & therefore #7 MUST go in Cell I3
4 1 4 3 5 2 7 6 8 9
5 8 7 9 1 6 3 4 2 5
6 5 6 2 8 4 9 7 1 3
7 3   4   1   8   6
8   2 1   3     7 4
9 6       5   2 3 1
A B C D E F G H I
1 4   58 3 7       2 Look at all # possibilities for each vacant cell…
2   3     9     4 8      i.e. the only #s that can go in Cell C1 are:
3     6   8   3   7           Look at all #s presently in Row 1 (4,3,7,2), Block AA (4,3,6) &
4 1 4 3 5 2 7 6 8 9               Column C (6,3,9,2,4,1)
5 8 7 9 1 6 3 4 2 5 Only #s 5 & 8 can go in Cell C1… (see red #s in C1)
6 5 6 2 8 4 9 7 1 3
7 3   4   1   8   6
8   2 1   3     7 4
9 6       5   2 3 1
A B C D E F G H I
1 4