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  • In this video, I'll introduce you

  • to a new way of computing division,

  • especially for larger numbers.

  • And then we'll think a little bit about why it works.

  • So we're going to try to compute what 96 divided by 4 is.

  • And I'm going to write it a little bit differently.

  • I'm going to write 96 divided by-- so I'm

  • going to write this strange-looking symbol

  • right over here, this thing that covers the 96.

  • But you could view this as 96 divided by 4.

  • And I'll show you in a second why we write it this way.

  • This is actually a very useful way of actually computing it.

  • So the first thing we'll do is we'll say, well,

  • how many times does 4 go into 9?

  • Well, we know that 4 times 2 is equal to 8 and that 4 times 3

  • is equal to 12.

  • So 3 would be too much.

  • We would go above 9.

  • So we want to be below 9 but not have too much left over.

  • We want the largest number that gets us

  • into 9 without going over 9.

  • So we'll say it goes two times.

  • 4 goes into 9 two times.

  • And then we say, what's 2 times 4?

  • Well, 2 times 4 is 8.

  • 4 times 2 is 8.

  • Or 2 times 4 is 8.

  • And now, we subtract.

  • We subtract the 8 from the 9.

  • And we get 1.

  • And now we bring down the next digit, which is the 6.

  • And then we ask ourselves, well, how many times

  • does 4 go into 16?

  • Well, in this case, we know that 4 goes into 16 exactly

  • four times.

  • 4 times 4 is 16.

  • So we say 4 goes into 16 four times.

  • Then we multiply 4 times 4 is 16.

  • We subtract.

  • And 16 minus 16, we have absolutely nothing left over.

  • And there we have our answer.

  • I know it seems kind of magical at this point.

  • But in a few seconds, we're going

  • to think about why this actually worked.

  • We got that 96 divided by 4 is equal to-- I

  • want to do that in a different color-- 24.

  • Now, what I want you to do right now is pause this video

  • and think about why did this actually work.

  • How did we magically get the right answer here?

  • And you can verify this.

  • Multiply 4 times 24, and you will get 96.

  • Well, I'm assuming you gave a go at it.

  • And the important thing always is

  • to keep track of the place value.

  • And it really tells you what's going

  • on when we do this process.

  • When we looked at this 9 right over here,

  • this 9 is in the tens place.

  • This is actually representing 90.

  • It represents 9 tens.

  • So we're saying, well, how many times

  • does 4 go into 90 if we're thinking about multiples of 10?

  • Well, it goes 20 times.

  • 4 times 20 is 80.

  • And so we said, well, 4 times 20 is 80.

  • But we still have 16 left over.

  • You do 96 minus 80.

  • You have 16 left over to divide 4 into.

  • And then 4 goes into 16 four times.

  • So really, a lot of this is just saying, well,

  • we first figured out that we could go 20 times.

  • And then we said, well, that doesn't get us

  • all the way to 96.

  • We have to go another four times.

  • Hopefully, that helps.

In this video, I'll introduce you

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A2 初級

數字除法:長除法介紹|四年級|可汗學院 (Dividing numbers: intro to long division | 4th grade | Khan Academy)

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    Yrchinese 發佈於 2021 年 01 月 14 日
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