退位减法
11-9=2
12-9=311-8=3
13-9=412-8=411-7=4
14-9=513-8=512-7=511-6=5
15-9=614-8=613-7=612-6=611-5=6
16-9=715-8=714-7=713-6=712-5=711-4=7
17-9=816-8=815-7=814-6=813-5=812-4=811-3=8
18-9=917-8=916-7=915-6=914-5=913-4=912-3=911-2=9
退位减法
11-9
12-911-8
13-912-811-7
14-913-812-711-6
15-914-813-712-611-5
16-915-814-713-612-511-4
17-916-815-714-613-512-411-3
18-917-816-715-614-513-412-311-2