Math Antics - Dividing Fractions

Hi and welcome to Math Antics. This video is all about dividing fractions,

But in order to understand how dividing fractions works,

we first need to learn about something called reciprocals.

A reciprocal is just a fancy math term for what you get

when you switch the top and bottom numbers of a fraction.

For example, if you have the fraction 1/2

and then switch the top and bottom numbers, you'll end up with 2/1.

2/1 is the reciprocal of 1/2 and 1/2 is the reciprocal of 2/1.

And an interesting thing about reciprocals is multiplying a fraction

by its own reciprocal will always give you one.

That's because you'll have the same multiplication problem on the top and bottom.

So you'll end up with a whole fraction, which is always one.

OK, that's nice.

But what do reciprocals have to do with dividing fractions?

Well, reciprocals let us do a really cool trick that makes dividing fractions easy.

Whenever you have to divide something by a fraction,

you can just multiply it by the reciprocal of that fraction instead

and you'll get the correct answer.

And that's great news because multiplying fractions is so simple.

This trick of multiplying by the reciprocal works

because fractions are really just mini-division problems.

So when you multiply something by 1/2,

it's the same as dividing by two, since two is below the fraction's division line.

And dividing by two is the same as dividing by 2/1 because you can turn

any number into a fraction by just writing a one as the bottom number,

right?

But look, reciprocals. That's why multiplying by 1/2 is the same as dividing by 2/1.

And it's true the other way around too.

So really,

it's kind of like you never have to divide fractions.

You can just rewrite your division problems so

that you're multiplying by the reciprocal instead.

Then when you multiply, you'll get the answer for the original division problem.

As always, let's see a couple examples of how this works

so you'll really understand.

Let's try this problem: 3/4 divided by 2/7.

OK,

so the first thing we want to do is rewrite our problem.

Instead of dividing by 2/7, we can multiply by the reciprocal instead.

The reciprocal of 2/7 is 7/2.

So our problem becomes 3/4 times 7/2.

Oh, I should mention a mistake that a lot of

students make when they first learn to divide fractions.

Sometimes students take the reciprocal of the first fraction,

the one that's being divided or even the reciprocal of both fractions.

But you only want to take the reciprocal of the second fraction,

the one you're dividing by.

OK, now that our problem has been changed to multiplication, it's easy.

Just multiply the tops, three times seven equals 21,

and multiply the bottoms, four times two equals eight.

And we have the answer to our fraction division problem.

So 3/4 divided by 2/7 is 21/8.

So that's pretty easy,

but let's try one more example. Let's try 15/16 divided by 9/22.

Again, the first thing we want to do is rewrite our problem.

We'll change the divided by 9/22 into times 22/9.

Now, all we have to do is multiply,

but since these numbers are kind of big, I'm going to use my calculator to help.

Let's see here.

So we have, all right,

on the top, we have 15 times 22 equals 330.

And on the bottom, we have 16 times nine equals 144.

So the answer to our division problem is 330/144.

Of course, that could be simplified for your final answer on a test,

but we cover simplifying fractions in another video.

All right,

that's how you divide fractions.

You just multiply by the reciprocal and you have your answer.

But there's one more thing I want to show you.

You already know that the line between the top and bottom number

of a fraction is just another form of the division symbol.

Well,

that means you'll sometimes see fraction division problems written like this.

This shows the top fraction, 2/3, being divided by the bottom fraction, 4/5.

It's really just that we have a fraction made up from other fractions.

The top number is a fraction and the bottom number is a fraction.

It just looks a little confusing because we have all these fraction lines here.

But we can make it look a lot better.

Let's rewrite this as a multiplication problem by

taking the reciprocal of the bottom number,

the fraction that we're dividing by,

and multiplying it by the fraction on top.

Tthere, that looks easier to do. And it's really the same problem.

We just need to multiply to get the answer.

So two times five equals 10

and three times four equals 12.

Ok. So there you have it.

What sounded really hard turns out to be as easy as flipping fractions upside down.

If you can multiply fractions, then you can divide fractions too.

Don't forget to practice what you've learned

by doing the exercises for this section.

Thanks for watching and I'll see you next time.

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