一、不一定要真的畫出物件再拿走
上一課我們學減法最直觀的意思：6Please respect copyright.ＰＥＮＡＮＡslR50ya2DX
> 有一個數量，取走一部分，看剩餘多少。
例如：6Please respect copyright.ＰＥＮＡＮＡT35ELX4Q1z
> 7 - 3
可以想像成有 7 粒糖果，吃了 3 粒，剩餘 4 粒。
但如果數字大一些，每次都要畫出所有物件再劃走，就會很慢。
所以今日我們學一個更方便的方法：6Please respect copyright.ＰＥＮＡＮＡfhMKDegwkz
> 倒後數 | Counting Back
即是：6Please respect copyright.ＰＥＮＡＮＡC5cuf5Q4dU
> 由原本數量開始，向後數取走的數量。
例如：6Please respect copyright.ＰＥＮＡＮＡdB91HLj7aH
> 7 - 3
由 7 開始，倒後行 3 步：6Please respect copyright.ＰＥＮＡＮＡPvWkgLI5jY
> 7 → 6 → 5 → 4
所以：6Please respect copyright.ＰＥＮＡＮＡj8sLldAask
> 7 - 3 = 4
二、倒後數為甚麼是減法？
減法：6Please respect copyright.ＰＥＮＡＮＡo1v7F9ff0u
> 8 - 2
意思是：6Please respect copyright.ＰＥＮＡＮＡcmkLdNK84Y
> 原本有 8 個，取走 2 個。
每取走 1 個，數量就少 1。
所以可以倒後數：6Please respect copyright.ＰＥＮＡＮＡUjkK8oraQo
> 8 → 7：取走第 1 個6Please respect copyright.ＰＥＮＡＮＡXAJhwhX21z
> 7 → 6：取走第 2 個
最後剩餘6Please respect copyright.ＰＥＮＡＮＡ2r9cO6bwLB
> 6
所以：6Please respect copyright.ＰＥＮＡＮＡ7gAxxvkqiA
> 8 - 2 = 6
主旨：6Please respect copyright.ＰＥＮＡＮＡUzPWVFwKDO
> 減多少，就由原本的數開始，向後行多少步。
三、用數線看倒後數
數線：6Please respect copyright.ＰＥＮＡＮＡmEmXHD9Ss2
> 0 —— 1 —— 2 —— 3 —— 4 —— 5 —— 6 —— 7 —— 8
做：6Please respect copyright.ＰＥＮＡＮＡRMOn5nKnDQ
> 8 - 3
由 8 開始，向左行 3 步：6Please respect copyright.ＰＥＮＡＮＡPEwDB26vkm
> 8 → 7：第 1 步6Please respect copyright.ＰＥＮＡＮＡrBNuBur8IE
> 7 → 6：第 2 步6Please respect copyright.ＰＥＮＡＮＡkcTEHgbexd
> 6 → 5：第 3 步
所以：6Please respect copyright.ＰＥＮＡＮＡXWAPYQwu79
> 8 - 3 = 5
留意：6Please respect copyright.ＰＥＮＡＮＡIPUCOxPH4w
> 起點 8 不是第 1 步。6Please respect copyright.ＰＥＮＡＮＡqCdVbrXGRl
> 減 3，是要行 3 步。
四、簡單例子
例子 1：5 - 1
由 5 開始，倒後行 1 步：6Please respect copyright.ＰＥＮＡＮＡdEYCFqeu1A
> 5 → 4
所以：6Please respect copyright.ＰＥＮＡＮＡ3AgxGsWNVB
> 5 - 1 = 4
這個就是：6Please respect copyright.ＰＥＮＡＮＡh4iEdF4OhX
> 5 少一是 4
//
例子 2：6 - 2
由 6 開始，倒後行 2 步：6Please respect copyright.ＰＥＮＡＮＡMJUnQJSpSH
> 6 → 5：第 1 步6Please respect copyright.ＰＥＮＡＮＡjPCEKQPEGT
> 5 → 4：第 2 步
所以：6Please respect copyright.ＰＥＮＡＮＡx4mQ3MCx6g
> 6 - 2 = 4
//
例子 3：9 - 4
由 9 開始，倒後行 4 步：6Please respect copyright.ＰＥＮＡＮＡNA1Iraj0tb
> 9 → 8：第 1 步6Please respect copyright.ＰＥＮＡＮＡkjWMV8V5sH
> 8 → 7：第 2 步6Please respect copyright.ＰＥＮＡＮＡojblXCC4aw
> 7 → 6：第 3 步6Please respect copyright.ＰＥＮＡＮＡL15AkSK6Xm
> 6 → 5：第 4 步
所以：6Please respect copyright.ＰＥＮＡＮＡv9T4cSyBWh
> 9 - 4 = 5
五、易錯的地方：數「數字」和數「步」不同
題目：6Please respect copyright.ＰＥＮＡＮＡyuyiAXuver
> 7 - 3
錯誤的做法可能是：6Please respect copyright.ＰＥＮＡＮＡusN1tTYDu0
> 7, 6, 5
然後回答：6Please respect copyright.ＰＥＮＡＮＡkZf7asZMVT
> 5
但這個是數了三個出現的數字：6Please respect copyright.ＰＥＮＡＮＡn8OHTgoHPN
> 7、6、5
真正要做的是：6Please respect copyright.ＰＥＮＡＮＡ40Ljk5qjdG
> 由 7 開始，行 3 步。
正確：6Please respect copyright.ＰＥＮＡＮＡBJp5SwChkU
> 7 → 6：第 1 步6Please respect copyright.ＰＥＮＡＮＡOSucH8G9l4
> 6 → 5：第 2 步6Please respect copyright.ＰＥＮＡＮＡaNU79PFwIv
> 5 → 4：第 3 步
所以：6Please respect copyright.ＰＥＮＡＮＡ4bADvmQK6N
> 7 - 3 = 4
記住：6Please respect copyright.ＰＥＮＡＮＡ3tYfBnAMg5
> 減 3 = 行 3 步，不是讀出 3 個數字。
六、倒後數和取走物件是同一件事
例如：6Please respect copyright.ＰＥＮＡＮＡrR50SanVMt
> 6 - 2
物件方法
有 6 粒糖果：6Please respect copyright.ＰＥＮＡＮＡy165hME1Eo
> 🍬🍬🍬🍬🍬🍬
拿去 2 粒：6Please respect copyright.ＰＥＮＡＮＡoM5Z8pmpNB
> 🍬🍬
剩餘：6Please respect copyright.ＰＥＮＡＮＡfZxS1ak5Ml
> 🍬🍬🍬🍬
所以：6Please respect copyright.ＰＥＮＡＮＡ4LOJ3PJx9n
> 6 - 2 = 4
//
倒後數方法
由 6 開始，倒後 2 步：6Please respect copyright.ＰＥＮＡＮＡSmJKrUKmvn
> 6 → 5 → 4
所以：6Please respect copyright.ＰＥＮＡＮＡiKQ6SaHKXi
> 6 - 2 = 4
兩個方法答案一樣，因為它們在描述同一件事：6Please respect copyright.ＰＥＮＡＮＡvXDryJNA45
> 每取走一個，就少一。
七、倒後數到 06Please respect copyright.ＰＥＮＡＮＡFS1PO0OaL2
如果你有 4 粒糖果，吃光 4 粒：6Please respect copyright.ＰＥＮＡＮＡnqW4H0sEkV
> 4 - 46Please respect copyright.ＰＥＮＡＮＡUfbNpmwdKr
由 4 開始倒後行 4 步：6Please respect copyright.ＰＥＮＡＮＡslqAv7Uyjd
> 4 → 3：第 1 步6Please respect copyright.ＰＥＮＡＮＡVpDotxm6oZ
> 3 → 2：第 2 步6Please respect copyright.ＰＥＮＡＮＡa7KZtkcKXp
> 2 → 1：第 3 步6Please respect copyright.ＰＥＮＡＮＡ5F2lZRk0Tn
> 1 → 0：第 4 步
所以：6Please respect copyright.ＰＥＮＡＮＡeDie7FSMZ1
> 4 - 4 = 06Please respect copyright.ＰＥＮＡＮＡhJQCbUt8zH
意思是：6Please respect copyright.ＰＥＮＡＮＡpqJBCnH6oE
> 全部取走，剩餘沒有任何一個。6Please respect copyright.ＰＥＮＡＮＡvLhNJPHcpI
八、減 0：一步都不行
如果是：6Please respect copyright.ＰＥＮＡＮＡZTEZhxDXlr
> 5 - 06Please respect copyright.ＰＥＮＡＮＡlxgIKsBprs
意思是：6Please respect copyright.ＰＥＮＡＮＡK32d1mpSwu
> 由 5 開始，倒後行 0 步。6Please respect copyright.ＰＥＮＡＮＡdtzrfrQfk9
所以仍然是：6Please respect copyright.ＰＥＮＡＮＡkSCh1PP463
> 56Please respect copyright.ＰＥＮＡＮＡv7M91XQeNB
因此：6Please respect copyright.ＰＥＮＡＮＡQJKdkIaXgK
> 5 - 0 = 56Please respect copyright.ＰＥＮＡＮＡANlRkViHGN
再例如：6Please respect copyright.ＰＥＮＡＮＡ52rAU9XVq0
> 9 - 0 = 96Please respect copyright.ＰＥＮＡＮＡKHGzVKBiiv
> 3 - 0 = 36Please respect copyright.ＰＥＮＡＮＡkRao4hcf8y
> 0 - 0 = 06Please respect copyright.ＰＥＮＡＮＡ3U60LX8WZY
核心意思：6Please respect copyright.ＰＥＮＡＮＡ7qNKjqguE7
> 減 0，數量不變。6Please respect copyright.ＰＥＮＡＮＡPalrnqBW52
九、暫時不倒後過 06Please respect copyright.ＰＥＮＡＮＡEiZFIRoouf
現在我們的數線暫時是：6Please respect copyright.ＰＥＮＡＮＡ1tEnvoNWZv
> 0 —— 1 —— 2 —— 3 —— 4 —— 56Please respect copyright.ＰＥＮＡＮＡnBsIDH7MWh
如果題目是：6Please respect copyright.ＰＥＮＡＮＡIeQSXBAWZV
> 3 - 56Please respect copyright.ＰＥＮＡＮＡpszKK5BHti
即是由 3 開始向左行 5 步。
但目前數線去到 0 就停：6Please respect copyright.ＰＥＮＡＮＡ1F4bK3yCKx
> 3 → 2：第 1 步6Please respect copyright.ＰＥＮＡＮＡUFMMwudUWD
> 2 → 1：第 2 步6Please respect copyright.ＰＥＮＡＮＡDQTnuRDu4t
> 1 → 0：第 3 步6Please respect copyright.ＰＥＮＡＮＡuj55KQeaut
之後還要行 2 步，但在現在「數量世界」入面，已經沒有得再少。
所以目前階段會話：6Please respect copyright.ＰＥＮＡＮＡvvnaxz8nIQ
> 3 - 5 暫時未能處理。6Please respect copyright.ＰＥＮＡＮＡ7c5d3KaIyf
將來學負數時，數線會向左延伸：6Please respect copyright.ＰＥＮＡＮＡ65nMMCRuBp
> -3 —— -2 —— -1 —— 0 —— 1 —— 2 —— 36Please respect copyright.ＰＥＮＡＮＡVlzbZIsdPy
到那時可以正式做：6Please respect copyright.ＰＥＮＡＮＡIogl75qzkb
> 3 - 5 = -26Please respect copyright.ＰＥＮＡＮＡoJ8rKNUqXl
十、故事題：用倒後數解減法6Please respect copyright.ＰＥＮＡＮＡy0gWhNBOVw
例子 4
你有 8 張貼紙。6Please respect copyright.ＰＥＮＡＮＡipIkA96wuy
你用了 3 張。6Please respect copyright.ＰＥＮＡＮＡ7eLsKqQJ8N
你剩餘多少張？6Please respect copyright.ＰＥＮＡＮＡc1EpHAwbWO
步驟 1：原本有多少？6Please respect copyright.ＰＥＮＡＮＡXLEHKQ7eb0
> 8 張6Please respect copyright.ＰＥＮＡＮＡ8MlpfNh3Y1
步驟 2：用了多少？6Please respect copyright.ＰＥＮＡＮＡVJGTrEumgD
> 3 張6Please respect copyright.ＰＥＮＡＮＡTJdK2d3z6r
步驟 3：寫成減法6Please respect copyright.ＰＥＮＡＮＡ3Syfiw5SZz
> 8 - 36Please respect copyright.ＰＥＮＡＮＡmQFCyf8pfg
步驟 4：由 8 倒後行 3 步6Please respect copyright.ＰＥＮＡＮＡpseYALpCaN
> 8 → 7：第 1 步6Please respect copyright.ＰＥＮＡＮＡi0RyCrczxq
> 7 → 6：第 2 步6Please respect copyright.ＰＥＮＡＮＡbXfKPpjRN0
> 6 → 5：第 3 步6Please respect copyright.ＰＥＮＡＮＡNcI1eVmFR2
所以：6Please respect copyright.ＰＥＮＡＮＡ2kmUGoAf4T
> 8 - 3 = 56Please respect copyright.ＰＥＮＡＮＡcEW6tjEiKz
答案：6Please respect copyright.ＰＥＮＡＮＡfnaKAkqR5N
> 剩低 5 張貼紙。6Please respect copyright.ＰＥＮＡＮＡ5cKp8IpwNT
十一、另一故事題：倒後數到 06Please respect copyright.ＰＥＮＡＮＡItvOtAvFIB
盒入面有 6 粒巧克力。6Please respect copyright.ＰＥＮＡＮＡbFLxW4jvqp
你吃了 6 粒。6Please respect copyright.ＰＥＮＡＮＡ48WmxqOeII
盒入面剩多少粒？6Please respect copyright.ＰＥＮＡＮＡocmzae185b
步驟 1：寫成減法6Please respect copyright.ＰＥＮＡＮＡf2ltzA7pWx
> 6 - 66Please respect copyright.ＰＥＮＡＮＡzAX24EML9v
步驟 2：倒後行 6 步6Please respect copyright.ＰＥＮＡＮＡZuymVmWrEN
> 6 → 5：第 1 步6Please respect copyright.ＰＥＮＡＮＡXG08FVoP6Q
> 5 → 4：第 2 步6Please respect copyright.ＰＥＮＡＮＡ5ESZMOv4y7
> 4 → 3：第 3 步6Please respect copyright.ＰＥＮＡＮＡ1ezGTtJLhf
> 3 → 2：第 4 步6Please respect copyright.ＰＥＮＡＮＡruzf9jteBq
> 2 → 1：第 5 步6Please respect copyright.ＰＥＮＡＮＡBuTih0CP22
> 1 → 0：第 6 步6Please respect copyright.ＰＥＮＡＮＡRPs0aMJgdd
所以：6Please respect copyright.ＰＥＮＡＮＡgOgw4j9wUj
> 6 - 6 = 06Please respect copyright.ＰＥＮＡＮＡcp0cfEHyjc
答案：6Please respect copyright.ＰＥＮＡＮＡQrCyMaC3tE
> 剩低 0 粒巧克力6Please respect copyright.ＰＥＮＡＮＡH5737mAIKh
十二、用倒後數檢查答案6Please respect copyright.ＰＥＮＡＮＡqu6nrfRHev
假設有人話：6Please respect copyright.ＰＥＮＡＮＡy0vGsas6H5
> 9 - 3 = 76Please respect copyright.ＰＥＮＡＮＡ6gmz92PQte
我們可以用倒後數檢查。
由 9 倒後行 3 步：6Please respect copyright.ＰＥＮＡＮＡy5eP9EtK0T
> 9 → 8：第 1 步6Please respect copyright.ＰＥＮＡＮＡFXIk453Eq6
> 8 → 7：第 2 步6Please respect copyright.ＰＥＮＡＮＡ1Ao0lRIg4i
> 7 → 6：第 3 步6Please respect copyright.ＰＥＮＡＮＡ4ktZMzWlu6
最後是：6Please respect copyright.ＰＥＮＡＮＡmscyDaDbW0
> 66Please respect copyright.ＰＥＮＡＮＡTJ4FjeTDEJ
所以：6Please respect copyright.ＰＥＮＡＮＡ5P6kjCYyUt
> 9 - 3 = 66Please respect copyright.ＰＥＮＡＮＡKrIFg3XXVy
原本的答案 7 錯了，因為只倒後行了 2 步：6Please respect copyright.ＰＥＮＡＮＡe3CoSp1fOL
> 9 → 8 → 76Please respect copyright.ＰＥＮＡＮＡSB93zyXx1m
要記住：6Please respect copyright.ＰＥＮＡＮＡWN2DgRiXEd
> 每一條箭嘴代表一步。6Please respect copyright.ＰＥＮＡＮＡqldKzI2PMt
十三、減法和加法可以互相檢查6Please respect copyright.ＰＥＮＡＮＡ3EvO1hgB42
如果：6Please respect copyright.ＰＥＮＡＮＡR5WvScAMzP
> 8 - 3 = 56Please respect copyright.ＰＥＮＡＮＡ7QFsyIduoO
那可以用加法檢查：6Please respect copyright.ＰＥＮＡＮＡ1vWd1ekeTO
> 5 + 3 = 86Please respect copyright.ＰＥＮＡＮＡnMvOuBR2AH
意思是：6Please respect copyright.ＰＥＮＡＮＡ2bhXFhodo7
> 剩餘 5，再加回取走的 3，就會回到原本的 8。6Please respect copyright.ＰＥＮＡＮＡj4wayVkC7e
所以：6Please respect copyright.ＰＥＮＡＮＡQvVSdsoRl4
> 8 - 3 = 56Please respect copyright.ＰＥＮＡＮＡ6ITyT95M0A
> 5 + 3 = 86Please respect copyright.ＰＥＮＡＮＡNNhsiRT0oY
這兩句是是相連的。
再例如：6Please respect copyright.ＰＥＮＡＮＡWPyerMN3dv
> 10 - 4 = 66Please respect copyright.ＰＥＮＡＮＡ7wchWEqtOB
可以檢查：6Please respect copyright.ＰＥＮＡＮＡfb99C3wCOQ
> 6 + 4 = 106Please respect copyright.ＰＥＮＡＮＡf2pnB0vZPa
如果加回之後回到原本數量，減法答案就合理。
十四、需要思考的例子：7 - 46Please respect copyright.ＰＥＮＡＮＡAPJi5LreTP
問題：6Please respect copyright.ＰＥＮＡＮＡJ2ccw3q2bQ
> 7 - 46Please respect copyright.ＰＥＮＡＮＡfT38NurUaQ
方法 A：倒後數6Please respect copyright.ＰＥＮＡＮＡbpHboLDGe2
由 7 倒後行 4 步：6Please respect copyright.ＰＥＮＡＮＡOQ9LbyyDXa
> 7 → 6：第 1 步6Please respect copyright.ＰＥＮＡＮＡy1UdhoRfH5
> 6 → 5：第 2 步6Please respect copyright.ＰＥＮＡＮＡ9iJi7rjg3w
> 5 → 4：第 3 步6Please respect copyright.ＰＥＮＡＮＡj70YQbjotP
> 4 → 3：第 4 步6Please respect copyright.ＰＥＮＡＮＡeWwYFSN9xI
所以：6Please respect copyright.ＰＥＮＡＮＡoBwERL9Ied
> 7 - 4 = 36Please respect copyright.ＰＥＮＡＮＡmeGTqbwuLW
//
方法 B：用加法檢查6Please respect copyright.ＰＥＮＡＮＡYfw4DYG7Sa
問：6Please respect copyright.ＰＥＮＡＮＡR6SuMwBuz1
> 3 + 4 是不是等於 7？6Please respect copyright.ＰＥＮＡＮＡtOrFz1G2gx
由 3 向前行 4 步：6Please respect copyright.ＰＥＮＡＮＡRy3M2e8FM0
> 3 → 4 → 5 → 6 → 76Please respect copyright.ＰＥＮＡＮＡ4U4Rj7kPl0
是，會回到 7。
所以：6Please respect copyright.ＰＥＮＡＮＡ08Ug5MtSvB
> 7 - 4 = 36Please respect copyright.ＰＥＮＡＮＡdOqTivpNLk
兩個方法互相支持。
十五、需要思考的例子：10 - 6
問題：6Please respect copyright.ＰＥＮＡＮＡxhBMDgWjOr
> 10 - 66Please respect copyright.ＰＥＮＡＮＡSSBdptXvit
方法 A：倒後數6Please respect copyright.ＰＥＮＡＮＡlTyw2IkHlt
由 10 倒後 6 步：6Please respect copyright.ＰＥＮＡＮＡZpX4A9mSNb
> 10 → 9：第 1 步6Please respect copyright.ＰＥＮＡＮＡJuLYb7EvOi
> 9 → 8：第 2 步6Please respect copyright.ＰＥＮＡＮＡxdke5DZqU4
> 8 → 7：第 3 步6Please respect copyright.ＰＥＮＡＮＡNH7k70R4MX
> 7 → 6：第 4 步6Please respect copyright.ＰＥＮＡＮＡfeD1BGJjTC
> 6 → 5：第 5 步6Please respect copyright.ＰＥＮＡＮＡ1o2w5E3HR4
> 5 → 4：第 6 步6Please respect copyright.ＰＥＮＡＮＡuyxPh57Sb6
所以：6Please respect copyright.ＰＥＮＡＮＡb5k65YJzwO
> 10 - 6 = 46Please respect copyright.ＰＥＮＡＮＡ1ntMyHxLeI
//
方法 B：用湊十的反向想法6Please respect copyright.ＰＥＮＡＮＡhoYRbek7IM
之前我們知道：6Please respect copyright.ＰＥＮＡＮＡowRtsssmIN
> 6 + 4 = 106Please respect copyright.ＰＥＮＡＮＡ3TX85JlTIS
所以如果整體是 10，取走 6，剩低就是：6Please respect copyright.ＰＥＮＡＮＡeqLEyTyFyv
> 46Please respect copyright.ＰＥＮＡＮＡd0nmvcTA86
因此：6Please respect copyright.ＰＥＮＡＮＡFecw5f5r1C
> 10 - 6 = 46Please respect copyright.ＰＥＮＡＮＡGDTeyL9mCW
這個開始接近「部分與整體」的想法：6Please respect copyright.ＰＥＮＡＮＡHxhplkPkb7
> 整體 - 一部分 = 另一部分6Please respect copyright.ＰＥＮＡＮＡH9ajV1em3z
十六、倒後數適合甚麼情況？6Please respect copyright.ＰＥＮＡＮＡDOzNWsgLnz
倒後數特別適合：6Please respect copyright.ＰＥＮＡＮＡqGwtBUYYzx
> 減的數比較小6Please respect copyright.ＰＥＮＡＮＡdhHLB9HsRB
例如：6Please respect copyright.ＰＥＮＡＮＡtRcuQjmh4E
> 9 - 26Please respect copyright.ＰＥＮＡＮＡvQQLjEDAHo
由 9 倒後 2 步：6Please respect copyright.ＰＥＮＡＮＡHXQyfUVNF8
> 9 → 8 → 7
很快。
但如果是：6Please respect copyright.ＰＥＮＡＮＡiw9GJAWjU1
> 15 - 86Please respect copyright.ＰＥＮＡＮＡC4RXTiQyaS
倒後數 8 步就會比較長。
之後我們會學另一種減法意思：6Please respect copyright.ＰＥＮＡＮＡ6GEUC2oMz0
> 減法：比較差距 | Subtraction as Difference6Please respect copyright.ＰＥＮＡＮＡKDML2Buknj
到那時可以用「由 8 數上去 15」的方法，可能會更快。
今日先集中掌握：6Please respect copyright.ＰＥＮＡＮＡqCCaxd4pss
> 取走多少，就倒後多少步。6Please respect copyright.ＰＥＮＡＮＡWRt3WUSPDY
十七、綜合例子：先加後減6Please respect copyright.ＰＥＮＡＮＡDXZaGzYV4g
你有 4 粒波子。6Please respect copyright.ＰＥＮＡＮＡ69HRPGSqyG
朋友送你 5 粒。6Please respect copyright.ＰＥＮＡＮＡ65I19QYhae
你之後送走 2 粒。6Please respect copyright.ＰＥＮＡＮＡWVfDbSfC6q
問題：6Please respect copyright.ＰＥＮＡＮＡ7ZycsElsMl
> 最後有多少粒波子？6Please respect copyright.ＰＥＮＡＮＡoB8iMw9yZO
步驟 1：先處理增加6Please respect copyright.ＰＥＮＡＮＡeJMe93GHXr
原本有 4 粒，朋友給 5 粒：6Please respect copyright.ＰＥＮＡＮＡBL9dJ6TBm4
> 4 + 56Please respect copyright.ＰＥＮＡＮＡhtjyzhaBuJ
由 5 開始數 4 步，或者由 4 開始數 5 步都可以。6Please respect copyright.ＰＥＮＡＮＡDS3zJtOAuF
用較快方法：6Please respect copyright.ＰＥＮＡＮＡlvALtB898B
> 5 → 6 → 7 → 8 → 96Please respect copyright.ＰＥＮＡＮＡYF0iuKadOW
所以：6Please respect copyright.ＰＥＮＡＮＡ6VAo41r5SR
> 4 + 5 = 96Please respect copyright.ＰＥＮＡＮＡvNo9W5Vvrc
現在有：6Please respect copyright.ＰＥＮＡＮＡ2Abq0H7bfZ
> 9 粒6Please respect copyright.ＰＥＮＡＮＡpwFRX30DM2
步驟 2：再處理取走6Please respect copyright.ＰＥＮＡＮＡ9T8dr7jvRq
送走 2 粒：6Please respect copyright.ＰＥＮＡＮＡAjVstFUL2q
> 9 - 2
由 9 倒後行 2 步：6Please respect copyright.ＰＥＮＡＮＡbUyzKl62Eu
> 9 → 8：第 1 步6Please respect copyright.ＰＥＮＡＮＡymsnHEgsAL
> 8 → 7：第 2 步6Please respect copyright.ＰＥＮＡＮＡWIayih5TBw
所以：6Please respect copyright.ＰＥＮＡＮＡ2MCTMkvZYD
> 9 - 2 = 76Please respect copyright.ＰＥＮＡＮＡkyKNDnz5Oq
答案：6Please respect copyright.ＰＥＮＡＮＡInt1awBCYx
> 最後有 7 粒波子。6Please respect copyright.ＰＥＮＡＮＡhVLz2FYVOn
十九、今日小結6Please respect copyright.ＰＥＮＡＮＡ6WWT7yG218
今課你掌握了：6Please respect copyright.ＰＥＮＡＮＡk7tZEhfBRF
1. 倒後數是做減法的方法。6Please respect copyright.ＰＥＮＡＮＡxalocFoJbB
2. 「a - b」可以理解成由「a」，向左行「b」步。6Please respect copyright.ＰＥＮＡＮＡmBjnHO823Q
3. 起點不是第一；每一次移動才是一步。6Please respect copyright.ＰＥＮＡＮＡ3GhZiFMXwN
4. 減 0，即是行 0 步，答案不變。6Please respect copyright.ＰＥＮＡＮＡ2aZgrc7YpK
5. 一個數減自己，會倒後到 0。6Please respect copyright.ＰＥＮＡＮＡx71QIk9pom
6. 減法可以用加法檢查：如果「a - b = c」，那「c + b = a」。6Please respect copyright.ＰＥＮＡＮＡ20yOfj0WK8
7. 目前階段倒後數不越過 0：負數會之後再學。6Please respect copyright.ＰＥＮＡＮＡi9IaAVzWRs
減法可以理解成由原本數量開始，沿數線向左一步一步倒後行。
ns216.73.216.133da2
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