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• - [Instructor] What we're going to do

• in this video is think about how

• we might multiply 592 times seven.

• And in general we're gonna think about

• how we would approach multiplying something

• that has multiple digits times something

• that has one digit.

• And the way we're going to do it is the way

• that, if you were to ask your parents,

• it's probably the way that they do it.

• And so the typical approach is

• you would write the larger multi digit number on top

• and then you would write the smaller single digit number

• below that, and since it's in the ones place,

• the seven, you would put it in the ones place column.

• So you'd put it right below the ones place

• in the larger number, so right below that two.

• And then you'd write the multiplication symbol.

• And the way you'd think about it is,

• all right, I'm just gonna take each of these places

• and multiply it by the seven.

• So, for example, if I'm taking those two ones

• and I'm multiplying it times seven,

• well that's gonna be 14 ones.

• Well there's no digit for 14.

• I can only put four of those ones over here.

• And then the other 10 ones I can express

• as one 10, and so I'd put it up there.

• Sometimes when people learn it,

• they say, hey, two times seven is 14.

• I write the four and I carry the one.

• But all you're doing is you're saying,

• hey, 14 is one 10 plus four ones.

• But then you move over to the tens place.

• You say, hey, what's nine tens times seven?

• Well, nine times seven is 63,

• so nine tens times seven is 63 tens,

• plus another 10 is 64 tens.

• You can only put four of those tens over here.

• So the other 60 tens you can express as six hundreds,

• so you can stick that right over there.

• Now a lot of people would explain that as saying,

• hey, nine times seven is 63, plus one is 64.

• Write the four and carry the six.

• But hopefully you understand what we mean by carrying.

• You're really trying to write 64 tens.

• Only four of those tens can be expressed over here.

• Or that's maybe the cleanest way to do it.

• And then the other 60 tens you can express as six hundreds.

• And then, last but not least,

• five hundreds times seven is going to be 35 hundreds,

• and then you add six hundreds, you get 41 hundreds.

• So 41 hundreds, so it's 4,144.

• Now I wanna reconcile this, or connect it,

• to with other ways that you might have seen this.

• So let's say that, let's do this again.

• So if we were to write 592 times seven.

• So one way that we've approached it in the past is

• we say, all right, what's two times seven?

• Well that's going to be 14.

• Notice that's the same 14.

• We're just representing it a little bit differently.

• Then we might say, well what is nine times seven?

• Gonna do the same color.

• And this really nine tens times seven.

• That's 63 tens, so you might write it right over there,

• which is the same thing as 630.

• And then you could think about

• what is five hundreds times seven?

• Well that's 35 hundreds, so you could write it like that.

• Same thing as 3500, and then you would add everything up.

• So you have a total of four ones.

• You have a total of four tens.

• You have a total of 11 hundreds.

• You could right 100 there.

• And then regroup the other 10 hundreds

• into the thousands place as 1000.

• 1000 plus three thousands is 4000.

• So we got the exact same answer because

• we essentially did the same thing.

• Over here, when we were carrying it,

• we were essentially regrouping things from here

• and you could think about it where we're condensing

• our writing versus what we did here.

• Here we just very systematically said two times seven,

• nine times seven, five times seven,

• but we made sure to keep track of the places

• to figure out what each of those,

• you could think of it as partial products, would be,

• and then we added.

• While here we carried along the way,

• essentially regrouping the values when we said,

• hey, two ones times seven ones,

• that's 14 ones, which is the same thing

• as four ones plus one 10.

• And so on, and so forth.

• So I encourage you, one, it's good to learn this method.

• It's the most common way that folks multiply.

• Once again, your parents probably learned it this way,

• but it's really valuable to understand

• why these two things are the same thing,

• so really ponder that.

• Think about that.

• And see if you can, if it all makes sense

• what's going on there.

• You're not just blindly memorizing the steps.

- [Instructor] What we're going to do

A2 初級

# 多桁の数字の標準的な掛け方の紹介 (Introduction to standard way of multiplying multidigit numbers)

• 40 1
林宜悉 に公開 2021 年 01 月 14 日