Kakuro Revisited

Contradiction Forcing

Assume a candidate is true and follow the forced steps. A conflict proves the candidate wrong.

Level 5 Last resort Technique 12

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Assume r8c5 = 1. Across 19 would then need 18 from the other two cells, but two different digits can make at most 9 + 8 = 17. The assumption fails, so r8c5 is not 1.

key digits removed

Why it works

When no direct technique applies, you can test a candidate. Place it as a hypothesis, then make only forced moves: last cell in a run, one candidate left, a run that has no partition at all.

If the hypothesis leads to a run that cannot be filled, a digit that has to go twice, or a sum that cannot be reached, the hypothesis was false. The candidate is removed. If every other digit has been removed, the last one is the answer.

The app proves this for you in the "Why not?" panel. It shows the assumption, the digits it forces and the exact conflict, so you can check every link of the argument.

How to spot it

  1. Pick a cell with two or three candidates and choose one.
  2. Place it and follow only forced moves.
  3. If you reach a conflict, remove the candidate.
  4. If there is no conflict and the puzzle still solves, the candidate is possible, but it is not proven correct.

Practice it

Open the app, choose a level that uses this technique, and ask the coach for the next step. Every hint links back to this guide.