Subtraction is the reverse of addition. We use the same complement idea but in the opposite direction.
Concepts clés
1 1) Direct subtraction
If the raised beads are available, lower them directly. Example 7 − 2: lower two earth beads → 5.
2 2) Five-complement
If there aren’t enough earth beads to subtract, raise the heaven bead first. Rule: subtract 5 and add the complement to 5. Example 7 − 3: since 3 = 5 − 2, raise the heaven bead (−5) and raise two earth beads (+2) → result 4.
3 3) Borrowing (ten-complement)
When the column isn’t enough, borrow 1 from the left column and add the ten-complement to the current column. Example 13 − 5: subtract 1 from the tens and add 5 to the ones → result 8.
Exemples interactifs
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