WEBVTT

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如果之前的練習你都做了

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並且已經記住了乘法表

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那麽你應該有這種感覺

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可能自己已經可以解決任何乘法問題了！

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你只需要記住--

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怎麽說呢--

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記住運算的一般步驟就可以了

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但是我不是要教大家做題步驟

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而是這種運算的本質和原理

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先來做一些

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你可能覺得自己不會的題目吧

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16×9

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16乘以9

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你可以會說

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老師 我沒有記住16的乘法表

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不可能會做這個題

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但是我要告訴你的是

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你一定會做！

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因爲我們可以把這個問題分解成

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你會做的題目

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到底怎麽做呢？

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首先是9乘以這個數的個位

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9×6

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這個大家肯定都會的

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我寫在這邊

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9×6=54

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知道乘法表就可以得出來

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那麽要寫下54來

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但是只能把4寫在個位這裡

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5是進位

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這就是運算的步驟

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在做加法時

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如果産生了大於9的數

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就要進位

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這種運算就是進位 沒什麽別的描述方法了

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然後9×1

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9乘以1

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這個很簡單

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9×1=9

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任何數乘以1都等於本身

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但是上邊還有個進位5

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9×1=9

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還要再加上5

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所以再加5

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得到什麽呢？

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9×1+5

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得14

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我寫在這裡

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14

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這樣就得出結果了

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16×9=144

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如果你一直記到12的乘法表的話

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應該要注意到144=12×12

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不過只知道9×6和9×1

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也可以解出這個難題

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現在你可能說 老師

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這不過是你耍的一個小把戲

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這怎麽可能呢？是爲什麽？

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大家應該經常這麽問

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不要只是被動的接受我說的話

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不要死記公式方法

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生搬硬套

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那麽 我換個寫法給大家解釋一下

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可以把16改成10--

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在這裡寫吧

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16=10+6

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把16拆開了

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當然9也可以拆

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不過只是一位數 可以留著

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就像這樣

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下面來看看這個問題究竟怎麽回事

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在這寫一個乘法號

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首先要用9乘以6

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你可能要問

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老師你爲什麽要這麽分16呢？

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其實我是把16的個位和

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十位拆開

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這個從右數的第二列

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這可不是個1 而是10

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16左邊的1是10 所以拆成10+6

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這就是這麽拆的原因

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好 現在來運算

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跟剛剛的方法一樣

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9×6--

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寫下來

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9×6=54

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但是這裡不寫54

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而是拆成50+4

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9×6=50+4

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好了 這一列代表個位

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畫一條虛線來分開

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這邊是個位

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只能寫一個4

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但還要寫50

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要找個地方寫50

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慣例是--

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至少我學的是這樣--

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把50寫在這上邊

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當然也可以寫在下面

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只要記得

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運算時得到十位有個50就行

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可以把50寫在這

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跟第一個題做的一樣

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只寫一個5

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剛剛就是只寫了一個5

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因爲這個5是在十位的

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實際上代表的是50

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十位的1就是10

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不過現在我要寫50

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這樣大家看的比較清楚 就代表50 或者10

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然後 9×10=？

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9×10

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這個我們也記過的

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任何數乘以10就是這個數後邊添個0

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得90

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9×10=90

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再加50

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90+50

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等於多少呢？

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是140

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9×10=90

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90+50=140

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爲了跟之前寫的保持一致

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可以把140寫成100+40

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所以我們要把40放在這裡

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百位進1

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但是100不知道放在哪

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可以寫在這

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可以--

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也可以寫在這

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寫在這裡

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很多地方可以寫

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這個100

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但是最重要的一點不要忘記

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這個一百是在百位上

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分開線我剛沒畫

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這樣再寫一個100

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得到是100+40+4

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即144

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希望大家能聽懂

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再來多做幾個題

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多看例題是很重要的

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計算55×8

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55×8

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差不多的例題

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要用8開始乘

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8×5

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我先寫下來

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8×5=40

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把0寫在這

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進位4

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然後又是8×5

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還是40

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不過再加上進位4 是44

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代表的是440

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你可以像我上個題一樣

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把55拆成50+5再分別乘以8

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不過做的題多了

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應該大家會習慣這麽運算的

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再做一個--

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換個顏色

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淺紅色

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用78乘以--

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乘以7

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8×7

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8×7=56

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寫下來--這又是一個新題

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8×7=56

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把6寫在下邊 進位5寫在上邊

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7×7=49

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7乘以7得49

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還要加上邊的5

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再加上5

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49+5=？

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是54

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7×7=49

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49+5=54

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得546

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不久前你可能還覺得

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光用1-12的乘法表

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肯定求不出78的乘法表

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但大家可以看到過程其實非常簡單

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再多做幾個

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一直算到不能再算爲止

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算到精疲力竭爲止

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89×3

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89×3=？

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3×9=27

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7寫在個位

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向十位進位2

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因爲27=20+7

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兩個10就是20

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加7是27

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然後3×8=24

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3×8=24

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上邊還有一個2的進位

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還要再加2

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得26

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3×8=24

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24+2=26

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結果是267

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再做一個

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不過我要加難度了

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在你覺得

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這些都會做的時候

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我再出一個能難到你的！

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239×6

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本來是要做

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兩位數乘一位數的影片的

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不過這題也是告訴大家

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不管是幾位數去乘一個一位數

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你都是可以做的

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步驟大同小異

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大家可以猜一猜這個要怎麽做

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6×9=？

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在這裡寫

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6×9

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這個之前算過了

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54

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把4寫在這

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5放在十位

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因爲這個54中的5就是5個10

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好

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然後是6×3

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6×3

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得18

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上邊還有一個5

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還要加5

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18+5=？

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6×3=18 18+5=23

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澄清一點

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並不是用6×3+5

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你看

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在這個問題裏

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3其實是30

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只不過寫的是3

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但這其實是6×30+50

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因爲39的3是3個10 或者是30

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那麽

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雖然這裡說6×3=18

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18+5=23

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實際得到的其實是230

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把3放在十位

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我換個顏色來寫

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用個跟上邊不一樣的

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結果是23

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把3放在十位

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向百位進2 寫上邊

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差不多就做完了 再乘最後一次

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6×2

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這個很簡單

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得12

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但是上邊還有進位2

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所以還要再加2

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+2

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等於什麽呢？

00:11:06.202 --> 00:11:08.130
12+2=14

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寫4

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6×2=12

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12+2=14

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把4寫在下邊

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如果這裡還有更高位的話就寫在上邊

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但是沒有了

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所以直接寫在這就可以了

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239×6

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=1434

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再做一個

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需要清一些空間出來

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既然已經增加難度了

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下面來做一個四位數的吧

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7362×--

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難點的--9

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9×2=？

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我就不再寫在旁邊了

00:12:00.494 --> 00:12:05.024
大家應該已經明白這個套路了

00:12:05.024 --> 00:12:07.200
9×2=？