Thousand Islands Life, Sudoku Puzzle #86 & #87

Can you believe it is the middle of July! Pick a rainy morning and give yourself a hearty Sudoku challenge.

Clueless?

As a bonus each month this year we will start with a Sudoku puzzle in progress, where it appears there are no more obvious or not-so-obvious clues. Does this puzzle #86 have any more clues.

(The answer follows below after the conclusion of Puzzle #87, the feature puzzle for July)

Feature Puzzle

The “Impossible” Series continues with Puzzle #87, the feature puzzle for July …

DAN’S 8-STEP APPROACH TO SOLVING ALL SUDOKU PUZZLES

Once you have completed the puzzle, to the extent that you have filled-in all obvious answers and have written all potential options across the top of the unsolved cells (PUZZLE PREPARATION), Dan recommends the following Steps to complete the puzzle.

Step 1: Sudoku Pairs, Triplets and Quads – See September 2015
Step 2: Turbos & Interaction – See October 2015
Step 3: Sudoku Gordonian Rectangles and Polygons – See November 2015
Step 4: XY-Wings & XYZ Wings – See December 2015
Step 5: X-Wings – See January 2016

Step 6: DAN’S YES/NO CHALLENGE
Step 7: DAN’S CLOSE RELATIONSHIP CHALLENGE
Step 8: AN EXPANSION OF STEP 7Steps 1-5 are relatively common techniques and are explained in the TI LIFE articles above. Steps 6-8 are covered in detail, in Dan’s book.

PUZZLE PREPARATION

Prior to utilizing Steps 1-8, complete the 5 Steps of Puzzle Preparation …

FILL IN DATA FROM OBSERVATIONS
FILL IN OBVIOUS ANSWERS
FILL IN NOT-SO-OBVIOUS ANSWERS
MARK UNSOLVED CELLS WITH OPTIONS THAT CANNOT EXIST IN THOSE CELLS
FILL IN THE OPTIONS FOR THE UNSOLVED CELLS

We will complete all of the first 4 steps in the order we observe them, until we conclude all Puzzle Preparation Step 1-4 clues.

The first thing we observe is that C8R7 & C9R7 have options 27.

Next, C3R7=5. And C1R6=5, C9R5=5, C7R1=5, C4R8=5 & C6R2=5. This concludes the 5’s.

In box three a 7 can only exist as an option in C8R2 or C9R2; therefore, C2R2 & C3R3 cannot have a 7 as an option.

Now the only cell in box 1 that can be a 7 is C3R3. C3R3=7. Then C5R5=7.

The options 27 in C8R7 & C9R7 prevent C4R7, C5R7 & C6R7 from having a 7 as an option; therefore, C4R9=7. C2R4=7

Now your grid should look like Example #87.1 below:

Look at box 2. In box 2 a 1 can only exist as an option in C5R1 or C5R2; therefore, a 1 cannot exist as an option in C5R7, C5R8 and C5R9.

In box 6 a 3 can only exist as an option in C8R5 or C8R6; therefore, a 3 cannot exist as an option in C8R1 or C8R2.
In box 7 a 3 can only exist as an option in C2R7 or C2R8; therefore, a 3 cannot exist as an option in C2R1, C2R2, C2R5 or C2R6.

Now you can see that in box 4 a 3 can only exist as an option is C3R5 or C3R6; therefore, a 3 cannot exist as an option in C3R1 or C3R2.

In box 5 a 9 can only exist as an option in C4R6 or C5R6; therefore, a 9 cannot exist as an option in C2R6, C3R6, C8R6 or C9R6.

In box 9 a 9 can only exist as an option in C7R7 or C7R9; therefore a 9 cannot exist as an option in C7R2, C7R3 or C7R4.

Now your grid should look like Example #87.2 below:

What do you notice about row 1?

The only cell that can be a 3 is C5R1. C5R1=3. Then, C6R7=3 and C2R8=3. Now C5R2=1, C6R9=1, C2R7=1, C3R1=, C1R4=1 C8R6=1, and C7R8=1.

From this point the puzzle is easily solved without advanced techniques, leading to a solution per Example 87.3 below:

The first time I tackled this puzzle I missed the clue of C5R1=3. I solved the puzzle with a Step 7 exercise. This is just one of many, many examples of how a Sudoku puzzle can be solved in different ways.

May the gentle winds of Sudoku be at your back.

Dan LeKander

Puzzle #86

Clue for Puzzle #86 … focus your attention on row 6. What do you observe?

There are only two unsolved cells in row 6 that can have options 12, which are C1R6 and C4R6.

Now the only unsolved cell in row 6 that can be a 3 is C5R6. C5R6=3.

This leads to C6R8=3, as well as additional answers.

Editor's Note: Summer 2021!

Dan and his proofreader (and, as they say, better half) Peggy, give us a new challenge each month.

It was back in January 2016, when we published a final article in Dan's Series of Steps to learn the logic of Sudoku – he asked if we would like a puzzle to solve every month … this editor said an enthusiastic… Yes, please!

If you have not already done so, I suggest you purchase Dan’s book: “3 Advanced Sudoku Techniques, That Will Change Your Game Forever!” Purchase of a book includes a 50-page blank grid pad, 33 black and two green tokens, to assist with Step 6.…