ubtract Fractions with Different Demoninators trick - mentally minus fractions instantly! - Free Educational videos for Students in K-12

ubtract Fractions with Different Demoninators trick - mentally minus fractions instantly! - By tecmath

Transcript


00:0-1	get a welcome to the tech Math channel . What
00:02	we're going to be having a look at in this
00:03	video is a way of subtracting fractions from one another
00:06	, fractions with , unlike denominators , different bottom numbers
00:09	and we're gonna be looking at how to do this
00:11	pretty much instantly . Okay , so it's a it's
00:13	without all that messy working out the often get with
00:15	fractions and once you work out how to do this
00:17	trick , look , that's a really , really great
00:20	and easy method of doing it as well . Okay
00:23	, so what about to give you an example ?
00:26	And I'm going to answer this one fairly quickly ,
00:28	but you see if you can answer it while I'm
00:30	busy writing it down . So three quarters , take
00:33	away one half . Now . The answer that you
00:36	should get with this is as follows , You should
00:39	get to over eight . Look and you can possibly
00:42	then simplify you can possibly you can simplify this further
00:45	into a quarter , but I'm going to leave them
00:47	unseen cliff . I'd from the minute . Okay .
00:49	So did you get that answer pretty much straight away
00:53	. If not , I'll tell you what I'm going
00:54	to show you how to do this pretty much straight
00:57	away . Okay . And I did that in that
00:59	exact speed . So I'll show you how to do
01:02	this . Yeah . As follows . This is how
01:06	we do it . So first off I'm gonna show
01:09	you how to get the bottom number . Then I'm
01:10	going to show you how to get the top number
01:11	bit . Once you start getting used to these ,
01:13	I tend to start working at the top number first
01:16	and then the bottom number . Okay , so the
01:18	bottom number um is pretty much easy . It's four
01:23	times to okay , so you just multiply these two
01:26	numbers , you're going to get your bottom number which
01:28	is eight . Okay , that's the first one to
01:31	get this number here is top number . What we
01:35	do is as follows , we're going to start and
01:37	this is a critical part here . We have to
01:39	start on this left hand side . And what we're
01:41	going to be doing is going to multiply multiply the
01:45	top number on the left hand side by this number
01:47	and then this number by this number and we're going
01:50	to be taking them away from one another . So
01:52	three times 2 is six , one times four is
01:57	four and I'm going to go six take away four
01:59	to get my answer which these two weights and it's
02:03	really , really critical that you actually do it in
02:05	that order that you don't go this one by this
02:08	one . Take away this one by this one because
02:09	otherwise you'll end up with four take away six and
02:12	you'll get the wrong answer . So we start in
02:13	the top left hand corner and we're multiplying this way
02:16	and then this way taking answers away . It's pretty
02:19	easy technique A Anyway , what about I give you
02:23	another example here , So This is a nice easy
02:27	one . What about 1/2 take away ? 2/5 .
02:34	So I'm going to do the working out with you
02:37	here . So let's first off get this bottom number
02:40	to get that . What do we do ? We
02:42	multiply these numbers . So two times five is ted
02:46	then what we did as we start in this top
02:49	left corner one times five is five , two times
02:54	two is four . Okay , we're taking that away
02:59	. So five take away four . We get the
03:01	answer of one . So our answer is 1/10 .
03:05	What I start to do after a while with these
03:08	is I'll start to actually work out this top number
03:10	first because that's typically the order in which we say
03:13	, we say it's 1/10 . So while you're busily
03:15	working this one out then you can automatically work this
03:18	one out for the bottom . Okay ? So and
03:20	that's the reason I'd start doing it in that order
03:22	. But whichever whichever way you like this , I'll
03:25	tell you what we'll do it one way like this
03:30	now . So 4/5 where I work at the top
03:33	number , so 4/5 take away two thirds . And
03:37	what does this equal ? So this is the last
03:39	bit of working out . I'm going to show you
03:41	that I'm going to start doing them in our heads
03:43	. So four times three is 12 And then two
03:47	times five is 10 . I'm gonna be taking that
03:51	away . Okay , so 12 take away 10 .
03:54	It's too And then five threes or 15 . So
03:58	we got to 15th . Did you go with that
04:03	? You might want to be pausing this while you're
04:04	doing this because you know , you might take a
04:06	little bit longer to start off with these . Okay
04:09	, a couple more of these . So what about
04:12	two thirds and from this , we're going to take
04:16	away one quarter . So let's start doing this without
04:20	that bit of working out . You might pause this
04:22	and do the working out . Maybe you will pausing
04:24	. Okay ? But I'm going to do this without
04:26	doing all that little working out down here . Now
04:28	, I'm just going to start going straight to the
04:30	answer . So what's going on in my head while
04:33	we're doing this is eight take away three , which
04:36	is five , three times 4 is 12 . How
04:43	did you go with that ? Pretty good . Good
04:46	. Okay , last question here , um 4/5 take
04:55	away half and once again we can start putting this
05:00	answer Australia , because we're going to take away five
05:02	, which is three , four times too late Take
05:07	away one times 5 , which is five it's true
05:09	. And the number here and on the bottom is
05:12	two times 5 , which is 10 , 3/10 .
05:17	Okay , pretty cool . Right . Really , really
05:20	easy method of doing these . And look , yeah
05:22	, look at that . There's no trying to change
05:25	to common denominators know what sort of deal , you
05:27	know , get this bottom number the same and there's
05:29	no multiple lines of working out . It's pretty much
05:32	there . Now look what we also haven't dealt with
05:35	here and I know there'll probably be a few people
05:37	are asked this and there are other videos I've done
05:39	on this is how to deal if you start to
05:41	get with mixed numbers , they say you have say
05:43	1.5 . Take away a quarter or a third or
05:46	something , how we would deal with that . And
05:48	there's just an extra little step when we do these
05:50	. So check out my other videos when we have
05:53	a look at these . The other thing which you
05:55	saw it started , the video was where we had
05:57	an answer which was to eighth . Now look that
06:00	that's it's a fairly raw answer . Um in a
06:04	maths class , your teacher often would like you to
06:07	simplify this and that's a fair enough thing . Okay
06:09	, so the way that we would simplify this is
06:11	we would look for a number that goes into both
06:13	the top and the bottom which is to one Two
06:18	goes into two once and two goes into 84 .
06:20	Okay . And that's where you can simplify these anyway
06:23	. Hopefully that video was of some help to you
06:26	. Hopefully you find that nice and easy anyway ,
06:28	see you next time . Bye .

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Ubtract Fractions with Different Demoninators trick - mentally minus fractions instantly! is a free educational video by tecmath.

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