1 00:00:02,200 --> 00:00:06,200 歡迎觀看負數的乘法和除法 2 00:00:06,200 --> 00:00:08,000 我們開始吧 3 00:00:08,000 --> 00:00:13,000 我想你會發現負數乘除法做起來比看起來要簡單許多 4 00:00:13,000 --> 00:00:15,300 你只需記住幾個規律就可以了 5 00:00:15,300 --> 00:00:18,300 並且我也會教你們爲什麽用這些規律 6 00:00:18,300 --> 00:00:22,200 首先,我要教給你們一些基本的規律 7 00:00:22,200 --> 00:00:33,000 這些規律就是,負數乘以負數的時候,比如-2x-2 8 00:00:33,000 --> 00:00:36,700 首先,你就只看這2個數字,不管前面的符號 9 00:00:36,700 --> 00:00:47,300 2x2就是4, 而負號x負號=正號 10 00:00:47,300 --> 00:00:58,700 我們寫下 規律1 負號X負號=正號 11 00:00:58,700 --> 00:01:11,300 如果是-2X2呢?同樣,我們也先不管數字前面的符號。2x2=4 12 00:01:11,300 --> 00:01:20,100 這裡有個負號X正號,結果負號X正號=負號 13 00:01:20,100 --> 00:01:30,000 規律2 負號X正號=負號 14 00:01:30,000 --> 00:01:35,900 那+2X-2怎麽辦呢? 15 00:01:35,900 --> 00:01:50,500 我想你大概已經猜到答案了,因爲這組乘法跟上面這組差不多 16 00:01:50,500 --> 00:01:54,900 2x-2也等於-4,所以我們總結這個規律 17 00:01:54,900 --> 00:02:02,500 規律3 正號x負號=負號 18 00:02:02,500 --> 00:02:11,100 實際規律2和規律3這2個規律是一個意思 19 00:02:11,100 --> 00:02:14,100 負號X正號=負號,正號X負號=負號 20 00:02:14,100 --> 00:02:18,100 你也可以說,數字前面的符號相反的時候,結果就是負數 21 00:02:18,100 --> 00:02:24,800 那可想而知正2X正2,答案就是正數了(4 ) 22 00:02:24,800 --> 00:02:31,400 我們回顧一下 負號x負號=正號,負號X正號=負號 23 00:02:31,400 --> 00:02:37,500 正號X負號=負號,正號X正號=正號 24 00:02:37,500 --> 00:02:43,600 最後這一部分是不是有點想不通呀?我試著把它簡化一下 25 00:02:43,600 --> 00:02:48,900 如果我告訴你,做乘法的時候 26 00:02:48,900 --> 00:03:12,300 數字前符號相同的話就得正號。符號相反的話,就得負號 27 00:03:12,300 --> 00:03:24,600 把上面的規律用數字表示就是 1x1=1, -1x-1=1 28 00:03:24,600 --> 00:03:39,400 1x-1=-1, -1x1=-1 看下面這組題,2個數字負號相反 29 00:03:39,400 --> 00:03:47,200 上面這2組題中,一組2個數字都是正號,一組2數字都是負號 30 00:03:47,200 --> 00:03:51,500 我們再來做幾道題,加深印象 31 00:03:51,500 --> 00:03:56,700 這樣你們做練習題的時候使用這些規律就更熟練了 32 00:03:56,700 --> 00:04:12,000 例如 -4x3, 4x3得12 33 00:04:12,000 --> 00:04:16,400 這2個數字的符號相反,一個負號,一個正號 34 00:04:16,400 --> 00:04:18,900 符號相反就得負號 35 00:04:18,900 --> 00:04:20,900 所以-4x3就得-12。這樣做是有道理的 36 00:04:20,900 --> 00:04:29,100 因爲這題還可以看成3個-4相加,-4+(-4)+(-4)也是得-12 37 00:04:29,100 --> 00:04:33,000 如果你看了我之前負數加減法的視屏就明白 38 00:04:33,000 --> 00:04:37,300 其實你應該先看那些視屏 39 00:04:37,300 --> 00:04:40,623 我們再做一題,-2 x -7 40 00:04:40,623 --> 00:04:42,200 有需要回顧前面內容的人 41 00:04:42,200 --> 00:04:43,477 可以隨時暫停視屏 42 00:04:43,477 --> 00:04:44,698 自己試試看是否掌握了這個技巧 43 00:04:44,698 --> 00:04:46,319 然後回頭接著看這題的答案 44 00:04:46,319 --> 00:04:52,300 2x7 得14,這2個數前面的符號是相同的 45 00:04:52,300 --> 00:04:58,400 那結果就是正14。正號一般省略不寫 46 00:04:58,400 --> 00:05:15,100 我想想,再比如9x-5, 9x5得45,數字前面的符號相反 -45 47 00:05:15,100 --> 00:05:30,300 最後這個例子我得想個好數字,恩,-6x-11吧, 6x11得66 48 00:05:30,300 --> 00:05:33,700 2個數前面的符號相同,那就得正66 49 00:05:33,700 --> 00:05:43,500 來個難點的吧,0x-12,盡管2個數前面的符號相反 50 00:05:43,500 --> 00:05:48,200 但0既不是正數、也不是負數 51 00:05:48,200 --> 00:05:54,200 0乘以任何數得0,不管對方是什麽數,結果都是0 52 00:05:54,200 --> 00:06:04,700 我們來看看這些規律是否也能用到除法上面來 53 00:06:04,700 --> 00:06:13,100 比如 9除以-3,首先我們只要想9除以3得多少 54 00:06:13,100 --> 00:06:19,500 答案是3 ,這2個數前面的符號相反 55 00:06:19,500 --> 00:06:28,900 符號相反得負號,所以答案是-3 56 00:06:28,900 --> 00:06:39,400 -16除以8呢?16除以8得2 57 00:06:39,400 --> 00:06:47,100 但是前面的符號是相反的,那結果就是-2 58 00:06:47,100 --> 00:06:51,600 記住 符號相反得負數 59 00:06:51,600 --> 00:07:02,000 -54除以-6呢? 60 00:07:02,000 --> 00:07:15,200 54除以6得9,而除數和被除數都是負數 61 00:07:15,200 --> 00:07:17,700 -54和-6,那答案就是正9 62 00:07:17,700 --> 00:07:24,400 記住 符號相同得正號 63 00:07:24,400 --> 00:07:33,700 再來一個例子0除以任何數等於0, 0除以-1, 答案等於0 64 00:07:33,700 --> 00:07:39,700 反過來呢,當然啦,0不能做被除數 65 00:07:43,550 --> 00:07:47,400 我再隨便舉個例子,4除以-1 66 00:07:47,400 --> 00:07:52,600 4除以1是4,但是符號相反 67 00:07:52,600 --> 00:07:55,600 所以答案是-4 68 00:07:55,600 --> 00:07:56,900 我想剛才的方法對大家有用 69 00:07:56,900 --> 00:08:02,700 現在我希望大家多找一些負數的乘除法練習題來做做 70 00:08:02,700 --> 00:08:06,100 你查看提示可以查找你需要用哪個規律 71 00:08:06,100 --> 00:08:14,300 有空的時候,你也可以想想爲什麽用這些規律 72 00:08:14,300 --> 00:08:17,100 有時間也可以想想負數乘以正數是什麽意思 73 00:08:17,100 --> 00:08:21,700 更有趣的是,你也想想負數乘以負數 74 00:08:21,700 --> 00:08:28,100 現在我希望你已經想自己做一些題了 75 00:08:28,100 --> 99:59:59,999 祝你好運!