Sudoku-YNext steps
Cage intersections: let a required digit block a row
You do not always need to solve a cage before it helps somewhere else.
Connect sums with positions
A cage sum restricts combinations; Sudoku units restrict their placement. If every surviving cage combination requires a digit, and every cage position for that digit lies in one row, that row already has its home for the digit somewhere inside the cage.
Killer Sudoku Online — Solving strategies · SudokuWiki — Cage Splitting
Our three-cell total of 8
Three distinct 9×9 digits totaling 8 have combinations {1,2,5} and {1,3,4}. Both require 1. Let the cage be D4, D5 and D6. Because all three cells lie in row D, remove candidate 1 from other cells in that row, even before settling its exact position inside the cage.
Do not remove 2 in the same way: the cage might use {1,3,4}, leaving 2 outside. Appearing in some combinations is different from appearing in every combination.
A cage can cross the row boundary
In a separate example, use D4, D5 and E5 for the same total. If another valid deduction rules out 1 at E5, the required cage 1 is restricted to D4/D5. The row elimination follows again. If E5 still allows 1, it does not: the cage could put its 1 in row E.
A practical checking order
List surviving combinations, find their common digits, then inspect every cage position for the digit you want to use. Only then choose outside targets. An incorrectly discarded combination can create a false “required” digit. Sudoku-Y does not currently scan this Killer pattern directly, so keep the reason available to revisit.
Try before looking
Try before revealing
A total-8 cage crosses rows D and E, with 1 allowed everywhere inside it. Can you remove outside 1s from row D?
Show explanation
No. The cage requires 1, but it could still put that 1 in row E. First establish that all its positions lie in row D.
References and further reading
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