EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 1, puzzle 99: 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 99 of 150 in Extreme Killer Sudoku 1
Difficulty
Expert 3 (expert and beyond), from the middle third of the book
Key technique
Finned Fish
Hard steps
9 of 86 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) (×3), Empty Rectangle (×2), Coloring/Multi-coloring, Forcing Chains, Multi-Sector Locking.

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 (9)
  1. Step 12 · Multi-Sector Locking advanced
    Rules out 8 from R1C1, R4C1, R5C1, R8C3; 9 from R1C1, R8C3.
  2. Step 34 · Empty Rectangle advanced
    Rules out 8 from R8C7.
  3. Step 35 · Empty Rectangle advanced
    Rules out 7 from R8C6.
  4. Step 36 · Advanced Innies (3+ sectors) advanced
    Rules out 2 from R1C5, R2C5; 4 from R1C5, R2C5; 5 from R1C5, R2C5, R2C6, R2C7 and 1 more.
  5. Step 40 · Advanced Innies (3+ sectors) advanced
    Rules out 5 from R8C5.
  6. Step 41 · Finned Fish expert
    Rules out 7 from R7C6.
  7. Step 42 · Forcing Chains expert
    Rules out 2 from R1C3.
  8. Step 57 · Advanced Innies (3+ sectors) advanced
    Rules out 3 from R8C5.
  9. Step 58 · Coloring/Multi-coloring expert
    Rules out 9 from R9C6.

The book

No starting digits at all: just cages and their totals. Sudoku rules, plus arithmetic, plus a lot of nerve.

150 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 Sudoku 2 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.