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歡迎大家

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今天我們來講負數的加減法

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讓我們開始吧

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首先，回顧一下，還記得什麽是負數嗎？

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我們還是來畫一條數軸

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哎呀，不太直。。。

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不過沒關係，你們明白就行

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好，我們都知道正數是怎麽回事兒了

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假如這是0

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然後是1,2,3,4 等等

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那2加2是多少呢，從2開始算

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你加上2，也就是往右邊走2步

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走到了4

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這個很自然，很簡單

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所以說

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2加2就等於4

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那麽，2減去1呢

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或者 3減去2呢

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我們從3開始，減去2

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往左走2步，就到了1

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好，我們計算了2加2等於4,3減2等於1

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你是不是覺得這太簡單了。。。

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那，如果1減去3是怎樣呢

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嗯

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其實道理是一樣的

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我們從1開始，往左走一步

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現在我們走到0以下了

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這是什麽意思呢？

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意思就是，我們就開始得到負數了

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負1，負2，負3，等等

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如果我從1這裡開始減去三

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向下走三步，停在-2這裡

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我們就會得到：1-3=-2

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其實我們在日常生活中

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經常會用到這樣的減法

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如果我說

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今天真冷，氣溫是1度

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而明天還會比今天還要冷3度

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你大概已經得到

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明天的溫度等於-2度了

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這個是常識

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這就是負數的意義

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你還要記住，一個負數的絕對值越大

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它的數值就越小

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比如-50少於-20

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因爲在我們的數軸上，-50比-20還要靠左

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這些你靠直覺是很容易的出來的

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當你一開始接觸負數的時候

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你可能會覺得，哦50比20大

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只不過，你要注意50和20都是負數

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讓我們來做道題吧

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我還是要用上這個數軸

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因爲它對我能起到很大的幫助

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我們來算算5-12

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你大概已經

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能估算到答案了

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但是我還是要畫個數軸

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從-10開始標上數字

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位置有點不夠

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我應該一開始就把它畫出來的。。

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就畫到5這裡

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讓我把這個箭頭重畫一遍，留點位置

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5-12

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讓我換個顏色

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從5開始，往左數12下

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因爲我們在減去12

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數一下，1,2,3,4,5.....

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我們停在了-7

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很好玩對吧？

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因爲12-5等於正7

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而5-12等於負7

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你想一想

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這是爲什麽？

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爲什麽5和12的差是7？

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而在這個情況下

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5-12的情況下

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我們會得到-7呢？

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更奇怪的是5和-7的位置差了這麽多？

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我剛剛說的似乎讓你混亂了。。

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不過我們再來試試另一道題吧

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從低一點的數字開始

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我們剛剛說過，5-12等於-7

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讓我們再做一道

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-3加上5等於幾？

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我們還是用這個數軸

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-3+5

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從-3這兒開始

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向上數5

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得到2

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所以我們的答案就是

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-3+5=2

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又有趣了，5-3也等於2

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既然我們看出來5-3和-3+5是一樣的

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我們就不難發現

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這兩個式子其實是一個意思

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當我們在運算負數的加減法的時候

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我們在做的只是很基礎的加減法

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只不過現在我們的答案

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可能會是0以下的數字了

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我們再來做一道吧

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我問你

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2減去-3等於幾？

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這種情況下，我們中間就有兩個減號

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你應該這樣看待這個問題

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中間這兩個減號

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負負得正，變成了+號

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所以我們會得到2+正3

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得到5

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我們再來做一道題

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-7減去-2等於幾？

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這樣，我們把中間的兩個減號負負得正

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在你的腦海浬想象一個數軸

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從-7向右挪2步

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會得到什麽呢？

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得到-5

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你再想想我們前面的例子

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這個也是一樣：-7+2等於-5，2-7也等於-5

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如果現在的溫度是2度

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明天比今天冷7度

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我們會得到-5度

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我們再來做幾道，熟能生巧嘛

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這些例子是很好的示範方式

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大概比我口述還要有用

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我們來使勁做題吧~

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-7減去3等於幾？

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我們從-7向左數三位

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我們會得到比-7小3的數字

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-10，對吧？

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你再想想前面的例子，如果是7+3

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我們從0上面的7

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向右三步

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我們會得到正10

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所以當你從0下面的7，向左三步

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你就會得到-10

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再來一道

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我知道你理解起來可能會有些困難

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不過練習是很好的幫助

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我們來算算3減去-5

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這兩個減號負負得正，變成+號

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我們就得到6了

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想想前面的例子，3-3等於幾？

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很簡單

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0對吧？

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那-3減去3呢？

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那就是比-3小3的數

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就是-6

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那-3減去-3呢？

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好玩吧

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兩個減號變成+號，

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我們從負3這裡開始，向右三步

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我們又得到了0

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能理解麽？

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我再來做一遍

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-3減去-3

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任何數減去它自己都應該等於0

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這就是爲什麽我們得到了0

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這同樣也是爲什麽

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這兩個減號

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變成加號了的原因

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我們再來做幾道題

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12減去13

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簡單吧？

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12減去12等於0,12減去13就是-1

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因爲我們從0又往左走了一步

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再來算算8-5

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這是個普遍的正數減法，等於3

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那麽5-8等於幾呢？

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我們先向左數五步到0

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再數3步，得到-3

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我在這裡再畫個數軸

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這裡是0，這是5

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看看，向左數了8步之後

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我們就得到3了

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你可以用同樣的方法計算任何這樣的加減法題目

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同樣也是很好的練習

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這一刻應該是個很好的入門了

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我建議你多多做練習

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因爲這些練習

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能讓你在腦海浬産生

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很好的鏡像印象

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比我在黑板上畫的任何東西

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都要來得有用

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所以，你可以試著再做做練習

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我去再找一些這樣的例子

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希望這樣你會對負數理解得更多

186
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你也可以參加我們的

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負數加減法專題討論會

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我希望你覺得這些東西很有趣

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拜拜~