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Full Analytical Solution for Difficult KS on Friday 2/17/17 https://www.calcudoku.org/forum/viewtopic.php?f=16&t=870 
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Author:  firefly [ Sun Feb 19, 2017 8:22 am ] 
Post subject:  Full Analytical Solution for Difficult KS on Friday 2/17/17 
Here's the Full Analytical Solution for the Difficult Killer Sudoku on 2/17/17. The starting puzzle: 1) F89 = [1,2] 2) Using known values for columns HI, H1+H7+H9 = 22. All must be 5+ and one must be 9. 3) The sixcell 23 cage at F6 is very telling. It must have 14 and cannot have a 9. 12 are blocked for F67 (from step 1) and H7 (from step 2). 4) With a 12 somewhere in G68, that blocks H9 from being 9, which means that H1 = 9. 5) G12 must be [6,7] because [5,8] would block all answers for the 14 cage at G4. 6) All cells in the 28 cage at F2 must be 4+, so G3 = [4,8]. 7) Using known values for the G1I3 block, H4 must be the same value as G3. 8) If both G3 and H4 are 4, then it would force G6 to be 8, which means that either H4 or G6 is 8. 9) With 8s and 9s blocked in the G4I6 block, the 10 cage at I45 must contain a 6 or a 7 and I6 must also be [6,7]. 10) With 6s and 7s blocked in column G and I, H7,9 must be [6,7]. After clearing up a bit, we have: It's honestly fairly easy from here on out. 11) With [1,2,3] in G68, F67 must be [4,7]. 12) Using known values for the D7F9 block, F7+D8 = 11, so D8 must also be [4,7]. 13) DE9 must be [3,9] because everything else is blocked by GH9. 14) E8 must be 6, and DE7 must be [5,8]. After some basic clearing, you have: 15) Using known values for the A7C9 block, B6+D8 = 12, but B6 can't be a 5, so B6 = 8 and D8 = 4. 16) That means that A7 must be 4, and after yet more clearing, we get: 17) Using known values for the D4F6 block, D6+F4 = 15, but 8s are blocked for both, so D6 = 9 and F4 = 6. 18) B45 must be [1,6] because 2s and 3s are blocked. 19) A56 must be [3,9] because everything else is blocked by A78. 20) C56 must be [5,7], C34 must be [3,4], F23 must be [5,9], etc. etc. It's all easy from here. 
Author:  eclipsegirl [ Wed Feb 22, 2017 3:58 am ] 
Post subject:  Re: Full Analytical Solution for Difficult KS on Friday 2/17 
When I did this puzzle, figuring out that H1 had to be 9 was the key. Of course, I spent a lot of time working other places with half starts and then getting blocked. 
Author:  paulv66 [ Wed Feb 22, 2017 12:51 pm ] 
Post subject:  Re: Full Analytical Solution for Difficult KS on Friday 2/17 
Thanks firefly. Having failed yet again to solve the Friday KS, I read the first part of your analysis. The fact that h1 equals 9 made everything else fall into place and I completed the puzzle this morning without looking at any more of your analysis. As expected, I got the message telling me that I got zero points as I didn't solve the problem in time. I noticed that the message has not yet been amended to state that you have two days to solve the Friday difficult KS. 
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