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  • Now that we know how to use fractions to represent parts of a whole,

  • there's a few different things we can do with them.

  • First of all, we can compare fractions.

  • Comparing fractions means checking to see if one fraction is

  • greater than, less than, or equal to another fraction.

  • That’s pretty easy if we think of fractions as parts of objects,

  • and draw pictures to help us see what we have.

  • Here’s an exampleWhich of these fractions is greater: three-eights or five-eights ?

  • To answer that question, let’s start by drawing two rectangles and divide them into eight equal parts.

  • Since the rectangles are divided into eight parts, each of those parts are called an eighth.

  • Now, let’s shade three parts of the first rectangleor three-eighths,

  • and five parts of the second rectangleor five-eighths.

  • Our picture makes comparing these fractions easy.

  • You can see that five-eighths is greater than three-eights,

  • because more of that rectangle is shaded.

  • Okay, now let’s try one that’s a little harder.

  • Which of these fractions is greater: three-fourths or four-fifths ?

  • Well, let’s start again with two rectangles, but this time,

  • we need to divide them up differently because the fractions we are comparing have different bottom numbers.

  • The first rectangle will be divided into four equal parts, and well shade three of them to show three-fourths.

  • The second rectangle will be divided into five equal parts, and well shade four of them to represent four-fifths.

  • Now to compare, all we have to do is see which rectangle is shaded in the most,

  • and that tells us that four-fifths is greater than three-fourths.

  • Therethat wasn’t so hard after all.

  • Alright, let’s try one more example. Let’s compare the fractions one-half and two-fourths.

  • Again, we start by drawing rectangle and dividing them up into parts: two on this one, and four on this one.

  • Next, we shade the parts of the rectangle according to our top numbers: one on this one, and two on this one.

  • Now, all we have to do is compare.

  • Well, what do ya know?

  • The same amount of each rectangle is shaded. That means these two fractions are equal!

  • It might seem strange that two fractions can have

  • totally different numbers and still represent the same amount, but they can!

  • Fractions like that are calledEquivalent Fractions’.

  • Equivalent fractions have different top and bottom numbers, but are equal in value.

  • Well learn more about equivalent fractions later in this video, but for now,

  • let’s find out what else we can do with fractions.

  • Another thing we can do with fractions is add them together.

  • Any two fractions can be added, but for now,

  • were only going to add fractions if they have the same bottom numbers,

  • because those fractions are much easier to add.

  • like these two fractions: one-fourth and two-fourths.

  • Let’s add them together.

  • Again, we can use drawings to help us solve this problem.

  • Looking at the rectangles for these two fractions,

  • we can add them visually just by rearranging the parts.

  • Because all of the parts arefourths’, our answer will also befourths’.

  • We can just take this one-fourth from over here, and combine it with these two-fourths,

  • and ta-dathree-fourths!

  • So, one-fourth plus two-fourths equals three-fourths.

  • Let’s try another one. Let’s add three-eights to five-eights.

  • We can use any shape we want, so I’m gonna use a circle this time.

  • So we have three out of eight here, and five out of eight here.

  • Just like our last problem, we can add these by combining the parts.

  • So let’s put these three over here with these five.

  • Well what do ya know?… That fills up all eight sections.

  • So three-eights plus five-eights equals eight-eights (or one whole circle).

  • In those examples, you might have noticed a pattern.

  • The bottom number of our answer was always the same as the bottom numbers of the fractions we were adding.

  • And the top number of our answer was just the sum of the top numbers of those fractions.

  • Well, that’s how it works.

  • That’s the procedure (or set of steps) for adding fractions that have the same bottom numbers.

  • That’s important, because if you can remember that procedure,

  • then you won’t need to use drawings to help you add fractions.

  • And that’s a really good thing, because what if you had

  • to add these two fractions together: fifteen-hundredths and ten hundredths.

  • It would be WAY too much work to draw rectangles and divide them up into 100 parts!

  • Fortunately, since we know the procedure for adding fractions, we can do it without the drawings.

  • First, let’s write out the problem.

  • Nowbecause were adding fractions, we know the answer will also be a fraction.

  • The bottom number of our answer will be the same as the bottom number of the fractions were adding: 100.

  • And the top number of our answer will just be the sum of our top numbers: 15 plus 10, which equals 25.

  • So as you can see, adding fractions with the same bottom numbers is easy when you know the procedure.

  • All of this brings up a really important point.

  • When youre first learning about fractions,

  • drawing pictures and imagining that fractions represent parts of cookies and candy bars can be really useful.

  • (And it can taste good too!)

  • Thinking of them that way can help you understand how simple fractions work,

  • and it can even help you solve some basic math problems.

  • But soon, youll have to do harder math problems!

  • And to solve those, youll need to stop thinking about fractions as just parts of things,

  • and start thinking about them in a different way.

  • And that’s what we are going to be talking about in the next section.

  • Before we move on, let’s review what weve covered so far.

  • We can draw pictures to show how fractions represent parts of a whole.

  • Using drawings, we can compare fractions to see which one represents the greatest amount.

  • If we compare two different fractions and find that they represent the same amount,

  • then we call them equivalent fractions.

  • We can also use drawings to help us do simple addition by combining the parts.

  • By doing this, we learned that the procedure for adding fractions that have the same bottom number,

  • is to just add the top numbers and keep the same bottom number in our answer.

  • Now to make sure you understand how to compare and add fractions visually,

  • be sure to do the exercises for this section.

  • learn more at www.mathantics.com

Now that we know how to use fractions to represent parts of a whole,

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B1 中級 美國腔

數學反常學--用零件工作 (Math Antics - Working With Parts)

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    Yassion Liu 發佈於 2021 年 01 月 14 日
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