EXTREMEPUZZLE BOOKS

Extreme Killer Sudoku 3, puzzle 123: 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 123 of 150 in Extreme Killer Sudoku 3
Difficulty
Expert 2 (expert and beyond), from the final, hardest third of the book
Key technique
Forcing Chains
Hard steps
10 of 99 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: Forcing Chains

Try each candidate of one cell in turn. Every choice leads to the same result, so that result must be true.

Other advanced and expert techniques in this solve: Advanced Innies (3+ sectors) (×4), Multi-Sector Locking (×2), 2-String Kite, Coloring/Multi-coloring, X-Wing/Swordfish.

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 · Multi-Sector Locking advanced
    Rules out 9 from R7C4, R7C6, R8C4.
  2. Step 14 · Advanced Innies (3+ sectors) advanced
    Rules out 6 from R3C4, R4C6, R5C6; 3 from R4C6, R5C6; 9 from R4C6, R5C6.
  3. Step 30 · Multi-Sector Locking advanced
    Rules out 9 from R9C2.
  4. Step 32 · Advanced Innies (3+ sectors) advanced
    Rules out 5 from R7C4, R8C4.
  5. Step 34 · Forcing Chains expert
    Rules out 3 from R3C5.
  6. Step 39 · Advanced Innies (3+ sectors) advanced
    Rules out 4 from R9C6; 5 from R9C6.
  7. Step 75 · Advanced Innies (3+ sectors) advanced
    Rules out 3 from R4C8.
  8. Step 77 · 2-String Kite advanced
    Rules out 7 from R9C8.
  9. Step 78 · Coloring/Multi-coloring expert
    Rules out 5 from R3C8.
  10. Step 80 · X-Wing/Swordfish advanced
    Rules out 7 from R5C1, R5C2, R5C8, R5C9 and 2 more.

The book

Volume three: 150 new killers, none repeated from books 1 or 2. 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.

On Amazon from 15 Dec About the book

Try a free sample puzzle (PDF) · prints on A4 or US Letter

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