WEBVTT

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既然我們稍微了解了

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正數和負數乘法的計算

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思考一下怎麽求解正數或負數的除法

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大家將發現

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這是個非常相似的方法

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如果它們都是正數

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得到正的答案 如果其中一個是負數

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或另一個 不全是負數 將得到負的答案

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如果它們都是負數

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負號抵消了 得到正的答案

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大家實踐一下 我建議大家暫停影片

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計算這些式子

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看看能不能求得

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跟我一樣的答案

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8÷(-2)

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如果是8÷2

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答案是4

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既然兩個數中有一個是負數

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就是這個

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答案將是負數

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所以8÷(-2)

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等於-4

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-16÷4

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大家要非常小心

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如果是

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16除以4

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答案是4

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因爲有一個數是負數

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兩個數中有一個是負數

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我將得到負的答案

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現在有

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(-30)÷(-5)

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如果是30÷5

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會得到6

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因爲是負數除以負數

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負號抵消了

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答案仍是6

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也可以在這裡寫正號

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但沒有必要

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這等於6

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負數除以負數

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就像是負數乘以負數

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將得到正的答案

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18除以2

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這是個有點陷阱的問題

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在談論負數之前

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大家就知道怎麽做了

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這裡是正數除以正數

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將得到正數

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將得到+9

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現在我們做些有趣的問題

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這是某種復合問題

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題中有乘法也有除法

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首先在這兒 根據題目寫法

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我們把分子乘出來

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大家不熟悉點號的話

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這是乘法的另一種寫法

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我可以將x寫在這兒

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大家將在代數學中看到

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點乘是非常常見的

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因爲X會有其它用處

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大家不想將它與字母X混淆

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X在代數學中很常見

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這就是爲什麽這裡使用了點

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這裡說

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分子是(-7)乘以3

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取這個數

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把它除以-1

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這個分子 -7・3

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+7乘以3等於21

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既然其中有一個是負數

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答案將是-21

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這將等於-21

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除以-1

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所以是

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(-21)÷(-1)

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負數除以負數得到正數

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這等於+21

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把計算法則都寫下來

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如果用正數除以負數

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將得到負數

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如果用負數除以正數

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將得到負數

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如果用負數除以負數

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將得到正數

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如果用正數除以正數

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將得到正數

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我們再做最後一題

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這裡全是乘法

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不過這很有趣

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因爲有三個數相乘

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我們還沒有做完

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可以按照從左邊往右邊的順序乘

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先思考一下

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(-2)×(-7)

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(-2)×(-7)

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它們都是負數

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負號抵消了 從這裡得到

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這個結果

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將得到+14

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要用+14做乘法

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乘以這個-1 乘以-1

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用正數乘以負數

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有一個是負數

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將得到負數答案

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我們將得到-14

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我再給大家一些題目

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一些有陷阱的題目

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如果有

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0÷(-5)呢

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這是0/(-5)

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0除以任何一個非零數

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將等於0

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如果倒過來呢？

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如果有-5÷0會怎樣呢？

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我們不知道

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除以0得到什麽

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我們還沒有定義

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有多種方法來概念化理解這個

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傳統上來說這是未定義的

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我們沒有定義

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某個量除以0時的結果

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同樣的 當0除以0時

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結果也是沒有定義的