Math Antics - Comparing Fractions - By mathantics
Transcript
00:03 | Uh huh . Hi there . Welcome to Math . | |
00:07 | Antics . In this video we're going to learn how | |
00:09 | to compare fractions . Mm This fraction has 25 more | |
00:14 | fiber than this fraction . But this fraction has try | |
00:18 | sodium phosphate . Well , it's not quite like that | |
00:22 | , comparing fractions just means telling which one is bigger | |
00:25 | . You know , just like we do with regular | |
00:27 | numbers when we use the greater than less than and | |
00:30 | equal to signs . That sounds easy . Right ? | |
00:33 | But unfortunately , unlike regular numbers , it's not always | |
00:36 | easy to tell which fraction is bigger just by looking | |
00:39 | at them . That's because the value of a fraction | |
00:41 | depends on both the top and bottom numbers and how | |
00:44 | they relate to each other . For example , if | |
00:47 | you have to compare these two fractions 1/3 and 1/10 | |
00:51 | . Some of you might be tempted to say that | |
00:53 | 1/10 is bigger because you know that 10 is bigger | |
00:56 | than three . Right ? But we need to remember | |
00:59 | that the fraction is really a number written like a | |
01:02 | division problem and its value depends on that division . | |
01:05 | So in this case the 1/3 is really the bigger | |
01:09 | fraction because its decimal value what you get when you | |
01:12 | divide is 0.33333 But the value of 1/10 is only | |
01:17 | 0.1 . Okay , so comparing fractions isn't quite as | |
01:22 | easy as comparing regular numbers but that doesn't mean it's | |
01:25 | going to be that hard . We're going to learn | |
01:27 | to methods for comparing fractions that make it very easy | |
01:31 | . The first method is called cross multiplying and it | |
01:34 | takes advantage of the fact that it's easy to compare | |
01:36 | fractions with the same bottom numbers . If two fractions | |
01:39 | have the same bottom numbers then we can just compare | |
01:42 | the top numbers . That's because we're comparing the same | |
01:45 | size parts were comparing 4th 2/4 8th , 2/8 10th | |
01:49 | 2/10 and so on . And the top number just | |
01:52 | tells us how many of those parts we have . | |
01:54 | So it's easy to see that 5/8 is more than | |
01:57 | 3/8 . But many times you'll have to compare fractions | |
02:00 | that have different bottom numbers or different sized parts . | |
02:03 | Fortunately there's a trick we can do to make the | |
02:06 | comparison easy in the math antics videos about common denominators | |
02:11 | . We learn a simple method for changing unlike fractions | |
02:13 | with different bottom numbers into like fractions with the same | |
02:17 | bottom number . Basically it shows how you can multiply | |
02:20 | two unlike fractions by whole fractions made from the different | |
02:23 | bottom numbers . So you end up with the same | |
02:26 | bottom number . This will give you two new equivalent | |
02:29 | fractions that you can easily add subtract or compare . | |
02:33 | But there's a shortcut for comparing fractions as long as | |
02:37 | we know that the bottom numbers of our fractions are | |
02:39 | the same . We don't really need to know what | |
02:41 | that number is . We just need to know what | |
02:44 | the top numbers will be since those are the ones | |
02:46 | that will actually compare . So instead of multiplying each | |
02:49 | fraction by a whole fraction , we can just multiply | |
02:52 | the top number of each fraction by the bottom number | |
02:55 | of the other fraction . This is called cross multiplying | |
02:58 | because if you draw a diagram of what you're multiplying | |
03:01 | it forms a criss cross pattern after you cross multiply | |
03:04 | you'll have two numbers . That would be the new | |
03:06 | top numbers . If you have made like fractions and | |
03:09 | those numbers will show you which fraction is greater . | |
03:12 | Let's try this cross multiplying method on an example or | |
03:15 | two . Let's compare the fractions 7/8 and 4/5 . | |
03:20 | We start by multiplying the second fractions . Bottom number | |
03:23 | five by the first fractions . Top number seven . | |
03:26 | And that gives us 35 for the new top number | |
03:29 | on this side . You'll always keep the answer on | |
03:31 | the side of the top number that you multiplied Now | |
03:35 | for the other side . The bottom number eight times | |
03:37 | the top number four gives us 32 for its new | |
03:40 | top number . Ha ha ! Now it's easy to | |
03:43 | see that the fraction 7/8 is greater than the fraction | |
03:47 | 4/5 because it's new . Top number , 35 is | |
03:50 | greater than the other new top number 32 . Let's | |
03:54 | do one more comparison by cross . Multiplying let's compare | |
03:57 | 6/11 to 9/15 . 1st will multiply 15 by six | |
04:02 | to get the new top number of the first side | |
04:04 | which is 90 . And you can always use a | |
04:06 | calculator to do the multiplication is if you need to | |
04:09 | Next we multiply 11 x 9 to get the second | |
04:12 | new top number which is 99 . So that tells | |
04:15 | us that the second fraction 9/15 is greater than the | |
04:19 | first fraction because it's new . Top number 99 is | |
04:22 | bigger . Pretty simple . Huh ? Okay . Cross | |
04:26 | multiply is pretty cool . But there's another way to | |
04:28 | compare fractions that you need to know about . But | |
04:30 | this one is only really good . If you can | |
04:32 | use a calculator , remember the reason that fractions are | |
04:36 | tricky to compare is because they're really division problems . | |
04:39 | But if we want to we can just do the | |
04:41 | division and get the answer which is the decimal value | |
04:44 | of the fraction . So if you have two fractions | |
04:46 | to compare you can just do the division , preferably | |
04:49 | using a calculator and then compare the decimal values . | |
04:53 | For example let's say I offered to give you either | |
04:55 | 5/12 of a pizza or 7/15 of a pizza . | |
04:59 | Now you happen to be really hungry so you want | |
05:01 | to choose the biggest amount but it's not very easy | |
05:04 | to tell just by looking which is bigger . 5/12 | |
05:06 | or 7/15 . This is where decimal values can really | |
05:10 | help you out . If you convert the fractions to | |
05:12 | decimals by doing division it will make it much easier | |
05:15 | to see which one's bigger . Five divided by 12 | |
05:18 | is about 0.42 and seven divided by 15 is about | |
05:22 | 0.47 yep . That makes comparing them much easier since | |
05:27 | 0.47 is greater than 0.42 It means that 7/15 is | |
05:32 | greater than 5/12 and that means that you'd rather have | |
05:35 | 7/15 of the pizza . Sometimes when you compare fractions | |
05:39 | this way you'll find two fractions that look different but | |
05:42 | have the same decimal value like 3/8 and 15/40 . | |
05:47 | If you convert each fraction to a decimal you'll see | |
05:50 | that they both have the value 0.375 two fractions that | |
05:54 | have different top and bottom numbers but the same value | |
05:57 | are called equivalent fractions . If two fractions are equivalent | |
06:01 | then you can just use the equal sign to show | |
06:03 | the comparison between them like this . All right . | |
06:07 | So those are two great methods you can use to | |
06:09 | compare fractions . Cross . Multiplying is simple and works | |
06:12 | great even if you don't have a calculator and comparing | |
06:15 | the decimal values by dividing is easy . If you | |
06:18 | do have a calculator as always practice . Makes perfect | |
06:22 | . So spend some time doing the exercises for this | |
06:24 | section and I'll see you next time learn more at | |
06:28 | math . Antics dot com . |
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