Fractions addition and subtraction trick - do them the fast way - Free Educational videos for Students in K-12

Fractions addition and subtraction trick - do them the fast way - By tecmath

Transcript


00:0-1	Good day and welcome to the Tech mouth channel .
00:01	In this video , we're going to be having a
00:03	look at as a way of adding two fractions together
00:06	. Ah , and that are subtracting two fractions from
00:09	each other where we have different denominators , different bottom
00:13	numbers . So I'll show you how to do this
00:15	. It's a really , really simple method and this
00:18	is the way I usually show my students score ,
00:20	saves a lot of hassle . So say for instance
00:22	, we have a fraction like this half and we
00:25	want to add to that a third to do this
00:29	. Really , really simply this is what we do
00:32	. Okay , so we hit eagles and I'll put
00:36	that line there . What we do is this to
00:43	get the bottom number First off , What we do
00:46	is we multiply these numbers . So two times three
00:50	is six . Pretty easy . Yeah . And then
00:53	what we do is we start at the top left
00:54	here and we multiply this number by the diagonal here
00:59	. Okay , so it's diagonal . Bottom number here
01:02	. So one times three it's three . We use
01:06	the same sign which is a plus . Then we
01:09	go to the right number here at the top .
01:11	And we multiply this number by this number one times
01:14	two . It's too Okay , equals three Plus 2
01:19	is five . And we got the bottom numbers of
01:22	six and there you go . We've got 56 That's
01:25	the easiest way of doing these . Okay ? Um
01:28	So anyway , I'll put another one up . Hopefully
01:30	you got that one . All right . I'll get
01:33	another another fraction . What about we go something a
01:36	bit different to go one quarter . And I'll put
01:39	what about something like 2/3 and that equals . All
01:44	right . Let's have a look . Get this pen
01:48	here . Yeah . And again we work at the
01:51	bottom number . We multiply these four times 3 is
01:55	12 . We get this number here the top left
02:00	. and we multiply it . Bye . This number
02:03	which is a three . Use the same sign multiply
02:08	this number By this one . Okay . two x
02:11	4 is eight . eight Plus 3 is 11 .
02:16	Both 12 . Yeah . You get that . Alright
02:20	. I reckon a few more examples because we're going
02:22	to do a few subtraction ones in a minute .
02:24	Also you'll notice with a few of these . Are
02:26	there there there there most simple . Um And so
02:29	a couple of these you can simplify a bit further
02:31	. Uh Let's have a look . We'll see if
02:33	we can get one . So what about if I
02:35	had one half to three quarters ? Hopefully this will
02:42	work . And what we do is again you might
02:46	give this one to go before I start . Yeah
02:50	you might have pause it and giving it a go
02:51	. Hopefully you did . So what we do is
02:53	we go two fours eight Start at the left .
02:58	It's really important to start the left . By the
02:59	way , I'm not starting over here . I'm always
03:01	starting over here . You'll notice one times 4 is
03:04	four . We use the plus three times 2 is
03:10	six . Okay ? So four plus six , it's
03:15	10 and then we get 10 8 . And what's
03:18	going to notice first off is that we can simplify
03:21	this further . So let's do this . First off
03:26	we look for a number that goes into both the
03:28	top and the bottom and you'll see straight away that
03:30	number is two . So we divide the top by
03:32	two . We get five , We divide the bottom
03:35	by two , we get four . We can still
03:38	simplify this . This is the moment what we call
03:40	an improper fraction . We can turn into a proper
03:43	into a mixed number that is . And the way
03:46	we do this is we go five divided by four
03:48	. Because if this big this top number is bigger
03:50	than the bottom number , You can do this .
03:52	five divided by four is one and one left over
03:55	, we put that one Over the bottom number ,
03:59	which is four . Okay . All right , then
04:03	. Another one . Okay . Hopefully these . Uh
04:07	not a problem , but hopefully you're picking them up
04:09	. Really , really easy , I must say .
04:11	I've always wondered why they teach fractions . Really difficult
04:14	, difficult . I often get a lot of students
04:17	come in and they've done fractions three or four times
04:19	in different in their school career , and they still
04:20	don't get it , and it's actually a little bit
04:22	annoying . Yeah , so anyway , I'll shut up
04:25	and I'll keep going . So what about we do
04:28	uh Three quarters and we more uh multiply . We
04:33	were not going to multiply , we're going to add
04:35	that Two a third . All right . What does
04:41	that equal ? Get the bottom number ? Three ,
04:44	fours 12 . Start up here . three , We
04:50	go plus 1 . 4 is four . We end
04:53	up with 13 , 3 plus four is nine plus
04:57	four is 13/12 . That we can simplify further because
05:01	we have this number here is bigger than this number
05:03	. And then so we divide 12 into 13 .
05:06	12 goes into 13 once , one reminder , one
05:10	over 12 . Okay , what about if we Start
05:18	with we could do the subtraction one . Okay ,
05:21	what about that ? We do 3/4 . Take away
05:27	. What about the 3rd ? Okay , it's the
05:30	same sort of thing . And this is where it
05:31	becomes really important that you start in a second .
05:34	Uh this top left in a little bit . So
05:36	first off we go 43 12 and this is where
05:39	this becomes important to start over here . nine threes
05:42	, 3 , threes are nine . Use the -14
05:46	is four . Do I take four is 5 ?
05:51	This goes over 12 . We have our answer now
05:53	. Just one little variation on this by the way
05:56	. Um I think this is a good thing to
05:57	get but I might even um I won't be able
06:00	to do this straight away but I'll put a link
06:02	up to this and this is where I'll be looking
06:05	at . I had a question that looks like this
06:08	uh Say we have something like 1.5 and we add
06:14	that two , two and a third . Okay ?
06:17	You notice now I'm not starting with simple fractions ,
06:20	I'm starting with mixed numbers . Okay . And we're
06:23	going to be adding mixed numbers . Mixed numbers .
06:25	Where we have a whole number and a fraction together
06:28	. So the way we deal with this is we
06:30	have to turn these into what is known as improper
06:32	fractions is is a fraction where the the number on
06:35	the top is bigger than the number on the bottom
06:37	. And the simplest way to do that as you
06:39	get this number , you times it by this number
06:42	and you add to this number that will give you
06:44	the number of the top . I'll get that again
06:46	to get the number of the top . This times
06:48	this plus this . So one times two is two
06:51	plus one is three . The bottom number stays the
06:55	same . The plus size aside Again . Same process
07:01	two times 3 or six plus one is seven .
07:06	And then the bottom number stays the same . And
07:08	then we treat it like we were before . Okay
07:11	, bottom number . What about we use red for
07:14	this ? Bottom No . two , threes six .
07:18	Okay . We do these ones three , threes and
07:22	nine . We put the plus there 7 to 14
07:28	. And then what we do is we add these
07:29	guys together 23 And we put that over six .
07:33	Now as you'll see again this number , This is
07:37	an improper fraction . This number is bigger than this
07:39	number . So we go 23 divided by six .
07:42	Because this line here means divided by 23 , divided
07:46	by six . It goes three times because three sixes
07:48	18 we have five left over five . We put
07:53	that over six . I hope that was of some
07:56	help anyway . Well , uh see you next time
00:0-1	.

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