EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 90: step-by-step solution

Step through the whole solution, or skip to the hard parts. Blue digits are placed by the walkthrough; red strike-through marks a candidate being ruled out.

Puzzle
Sudoku, number 90 of 150 in Extreme Sudoku 2
Difficulty
Expert 5 (expert and beyond), from the middle third of the book
Key technique
XY-Chain
Hard steps
15 of 42 steps need an advanced or expert technique

Blue digits are placed by the walkthrough. Red strike-through marks a candidate being ruled out. Cells are named by row and column: R3C5 is row 3, column 5. Arrow keys step too.

The key technique: XY-Chain

A chain of two-candidate cells, each linked to the next. Whichever way the start cell goes, one end of the chain holds the digit, so cells seeing both ends cannot.

Other advanced and expert techniques in this solve: W-Wing (×2), ALS-XZ, Cell Forcing Chains, Finned Fish, WXYZ-Wing, XY-Wing.

Every move above the basics is listed on the sudoku techniques page, each with a one-sentence explanation and worked examples from the books.

The hard steps, in text (15)
  1. Step 7 · Finned Fish expert
    Rules out 8 from R5C7.
  2. Step 8 · ALS-XZ expert
    Rules out 2 from R9C9.
  3. Step 9 · Cell Forcing Chains expert
    Rules out 7 from R5C1.
  4. Step 14 · W-Wing advanced
    Rules out 2 from R6C5.
  5. Step 15 · XY-Chain expert
    Rules out 2 from R7C4, R7C5, R7C6, R9C7.
  6. Step 16 · XY-Wing advanced
    Rules out 2 from R6C8.
  7. Step 17 · XY-Chain expert
    Rules out 6 from R6C5.
  8. Step 19 · XY-Chain expert
    Rules out 4 from R8C4.
  9. Step 20 · XY-Chain expert
    Rules out 2 from R4C2.
  10. Step 21 · XY-Chain expert
    Rules out 4 from R7C7, R7C8, R7C9.
  11. Step 23 · XY-Chain expert
    Rules out 4 from R8C1, R8C7.
  12. Step 24 · WXYZ-Wing expert
    Rules out 2 from R4C3, R6C4, R6C6.
  13. Step 30 · W-Wing advanced
    Rules out 1 from R7C4.
  14. Step 31 · XY-Chain expert
    Rules out 6 from R1C7, R1C9, R3C2.
  15. Step 37 · XY-Chain expert
    Rules out 7 from R4C2.

The book

Volume two: 150 new classic 9×9 grids, none repeated from book 1, ramping from hard to brutal.

150 new puzzles. Expert and beyond. Puzzles by M. J. Bailey. Paperback, 5 × 8 in, 196 pages, £9.99 / $11.99. Every solution in the book carries a QR code to its own walkthrough, so you can check one step without seeing the whole answer.

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More step-by-step solutions: Extreme Sudoku 1 · Extreme Killer Sudoku 1 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.