EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 2, puzzle 9: 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
Killer sudoku, number 9 of 150 in Extreme Killer Sudoku 2
Difficulty
Expert 3 (expert and beyond), from the opening third of the book
Key technique
Finned Fish
Hard steps
10 of 58 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: Finned Fish

An X-Wing or Swordfish with one extra candidate (the fin). Cells that are in the fish pattern and also see the fin lose the digit.

Other advanced and expert techniques in this solve: Advanced Innies (3+ sectors) (×2), Empty Rectangle (×2), Multi-Sector Locking (×2), Coloring/Multi-coloring, Forcing Chains.

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

The hard steps, in text (10)
  1. Step 13 · Advanced Innies (3+ sectors) advanced
    Rules out 9 from R1C7, R2C7, R4C9, R6C7; 5 from R3C9, R5C7, R5C9, R6C9; 6 from R3C9.
  2. Step 32 · Empty Rectangle advanced
    Rules out 9 from R9C1.
  3. Step 33 · Multi-Sector Locking advanced
    Rules out 7 from R1C6, R2C6; 9 from R3C4; 3 from R6C1.
  4. Step 34 · Advanced Innies (3+ sectors) advanced
    Rules out 4 from R3C4.
  5. Step 35 · Coloring/Multi-coloring expert
    Rules out 8 from R3C5, R3C6.
  6. Step 36 · Empty Rectangle advanced
    Rules out 8 from R6C1.
  7. Step 37 · Finned Fish expert
    Rules out 8 from R5C1.
  8. Step 38 · Finned Fish expert
    Rules out 9 from R1C6, R2C6.
  9. Step 41 · Multi-Sector Locking advanced
    Rules out 9 from R1C3.
  10. Step 42 · Forcing Chains expert
    Rules out 7 from R1C3.

The book

Volume two: 150 new killers, none repeated from book 1. No starting digits, just cages and their totals, 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 Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.