ast Mental Multiplication Trick - multiply in your head using base 10 - Free Educational videos for Students in K-12

ast Mental Multiplication Trick - multiply in your head using base 10 - By tecmath

Transcript


00:01	get and welcome to the Tech Math channel . This
00:03	video is the first video in a series of videos
00:05	, we're going to be having a look at the
00:07	base . Method of multiplication is a method of multiplication
00:10	. Multiplying bigger numbers where you can start to mentally
00:13	multiply bigger numbers . Ah using this idea of a
00:17	base now , what is this ? It's a base
00:19	is an easy method of multiplication . We're basically ,
00:23	what you're doing is you're multiplying two numbers that we're
00:25	going to start out . They're gonna be fairly close
00:27	to one another , but they're gonna get further and
00:28	further away . We're gonna multiply these two numbers using
00:31	a third number , a nice easy number which we
00:34	call the base . Uh And you'll see how this
00:37	works . So few . You guys will be able
00:39	to multiply some of these numbers Quite easy . You've
00:41	seen my earlier videos , but this video's was just
00:45	starting um this method of multiplication , which we will
00:48	build upon . So it's really , really worth sitting
00:50	through and watching because you I do modify the way
00:53	I can do this multiplication . So First off ,
00:56	I'll give you an example . You should be able
00:58	to this fairly quickly . What's 14 Times 12 ?
01:03	Now look , you're pretty much automatically should get the
01:07	answer come to your head that this is 168 .
01:11	Okay . If you didn't don't worry if you did
01:13	. Congratulations . But I'm going to show you the
01:15	way that we work using this basement and now base
01:18	number you're gonna see these numbers are quite close to
01:21	one another . First off they do have a number
01:23	which is fairly much in common in this number is
01:25	a base number of 10 . Ten's a number which
01:29	is nice and close to these numbers . And 10
01:30	is also a pretty easy number to multiply around with
01:34	other numbers which we can use as basis you'll see
01:36	in future videos to include things like 20 maybe 50
01:39	Even 100 . And they can go even further from
01:41	there . Okay , so these are nice easy numbers
01:44	to multiply with me . I'll show you how we
01:46	play around with his base numbers . So we've used
01:49	were chosen the base number of 10 . So when
01:51	we look at 14 , this 14 is four more
01:56	plus four more than their base number here . So
01:59	plus four and 12 is plus two more Then our
02:05	base number 10 . So the first way we get
02:07	our answer is this we add Either this number to
02:12	this number so to here to the 14 or we
02:16	could add The 4 to the 12 there . You
02:19	see that . So we do a bit of cross
02:21	edition sort of thing here and you're going to see
02:23	that's going to give us the same answer no matter
02:24	what . This is often the case . This is
02:26	actually always the case when we do this , so
02:28	you just have to choose one of them to add
02:30	. Okay , so first off let's add 14 To
02:37	to hear , we get the answer of 16 .
02:40	Yeah , that's how we get our first part of
02:42	our answer . The next thing we do is we
02:45	multiply These two numbers together . Okay . This plus
02:48	four and plus two . Okay . It's what do
02:51	you get when you multiply plus four plus two ?
02:53	When you multiply four and two , you get the
02:55	number eight . So We get the answer of 168
03:00	. Okay , you're pretty good with this . Okay
03:03	, so we're gonna explore this a little bit more
03:05	. Okay , I'll put another example up . That's
03:10	really , really important . You get this idea of
03:12	first off which base we're using also how we're getting
03:15	those numbers above those particular numbers . So let's let's
03:18	give you an answer here an answer . Let's give
03:19	you a question here . What about we do this
03:21	one 15 times 13 . Now again , we're only
03:27	gonna be playing around with a base 10 in this
03:29	particular video , but we will use other bases in
03:31	other videos . So using base 10 , This is
03:35	five above 10 and this is three above 10 .
03:38	So we add one of these numbers together with us
03:41	. Either this one or this one , it doesn't
03:43	matter again . So 15 plus three . We get
03:47	a line . Now when you're multiplying these mentally ,
03:50	I actually find it helpful . Do not think of
03:52	this as a team , but it's actually 180 .
03:54	Okay , That's a 10s part . The addition part
03:58	gives us our first part and then we made these
04:00	guys together . So five Times three is 15 and
04:05	we're going to add these guys together . So 180
04:07	plus 15 Is a nice easy one . You can
04:10	do in your head , which is 195 . Okay
04:13	. I'm only writing those . They have only just
04:14	so we can show what's going on . I think
04:16	these are really easy ones to do in your head
04:19	. You see that ? Okay . I'll give you
04:22	another example . This is really critical that you get
04:25	the basics of this if you decide to continue on
04:27	with this . So what about we do another one
04:29	. We'll do 17 times 12 Again , we're using
04:35	base 10 . We're gonna go plus seven plus two
04:41	. We add these two numbers together . We get
04:42	100 we at 19 , which is we're seeing as
04:45	190 . We multiply these two numbers we get 1490
04:49	plus 14 is 204 . Okay , how do you
04:55	go with those ? Pretty good . I'll tell you
04:57	what , I'm gonna modify these a little bit .
04:58	So uh and you'll see this modification in a bit
05:01	. Now again , there's some of these questions you
05:03	may be able to answer fairly easy using different methods
05:06	, but I'm gonna encourage you just from the minute
05:07	to use this particular method because it is going to
05:09	get tricky , it's going to get well trickier so
05:12	much , but it is going to start looking at
05:14	different paces . So what about we do this one
05:17	? What about we go eight Time 17 because we
05:20	can also You're going to notice these numbers are close
05:23	to 10 to so we can always so use these
05:25	numbers as well when we're using a base tan ,
05:27	but there's a little bit of a difference this time
05:29	. Okay . Yeah , this number here is two
05:34	below . Okay , I'm going to use a different
05:37	colour to show that obviously in your head you won't
05:39	be able to do that . So this is to
05:41	below 10 and this one here his seven above and
05:46	again , the same thing . You can either 17
05:48	plus minus two or eight plus seven . It gives
05:50	you the same answer of 15 or 150 . Then
05:55	we multiply these guys together now -7 . That -2
05:59	times seven Is not 14 , it's -14 . Okay
06:05	. So we got to take 14 away from our
06:06	answer And we get the answer of 136 . Okay
06:14	. You see how that worked ? These two guys
06:16	give a negative number . So we're actually taking our
06:18	answer away here . And it's a little trick that
06:21	, look , it's easy to fall for occasionally when
06:22	you are first doing these . Okay , what about
06:26	I'm going to give you I think an answer the
06:28	question they're going to know the answer to almost straight
06:30	away but let's investigate it anyway . What about we
06:33	do nine times nine now look hopefully you're looking at
06:35	that straightaway going yeah I know the answer to that
06:37	and hopefully you're gonna say it's 81 but let's have
06:40	a look at this using this method . So this
06:43	is one below 10 . This is one below 10
06:46	. So nine take 1 . Yes I'm going to
06:50	say there's 80 then minus one times minus one now
06:54	minus times minus gives us a positive number . So
06:56	you know get plus one . Okay i . d
07:00	. plus one is 81 . Cool . Yeah We
07:07	can even go a little bit further than this .
07:08	We can we can actually do numbers almost on the
07:11	other part of the spectrum . I'm gonna give you
07:12	a couple examples in a second , a couple of
07:14	your own ones to work through . We can even
07:15	do something like say this 1 19 times 21 .
07:21	Okay . We can use this idea of basis here
07:23	. This number here is nine above 10 . This
07:25	one here is a bit of a plus there .
07:27	This one here is 11 above 10 , so 19
07:31	plus 11 is 30 becomes 300 And then 11 times
07:36	nine is 99 . Your answer , you got it
07:41	, It's 399 . But what you might notice with
07:46	this is this numbers not only these numbers are not
07:48	only sort of close to 10 , but they're actually
07:50	closer to 20 . Okay . We can actually use
07:53	base 20 to work things out and that's what we're
07:55	gonna be having a look at in our next video
07:57	is the idea of using base 20 . Okay ,
08:00	now look , I think I'll leave you with a
08:01	few examples with this just to consolidate this . But
08:04	first off what you probably should get with today's video
08:06	with this part of the video is this idea of
08:08	using these numbers here ? This idea of using a
08:11	base Okay . And how we deal with these when
08:14	we are doing them okay because it actually doesn't vary
08:17	a huge amount even when we start getting even bigger
08:19	numbers . Or I only give you a couple of
08:23	examples here . See here you go with them .
08:24	What about we do these ones 14 times 13 .
08:28	What about we do ? I'll do 19 times 18
08:33	. What about we do 15 times 15 . Okay
08:39	, pause the video and see how you go with
08:41	answering these . Okay , how did you go ?
08:49	Let's have a look . So this one's four above
08:51	This . One's a plus three . So we get
08:54	14 plus three , which is 1 70 . Okay
08:58	, 17 70 . 1 70 . And then we
09:00	multiply these two numbers . Four times three is 12
09:05	. So we add these guys together and we get
09:07	the answer of 182 . Okay . What about we
09:14	do this next one ? Now we get this one's
09:18	right above This one here . 9 is one below
09:23	, so No one plus idle Idling Tai Kwan is
09:28	79 So again 170 . And then we multiply .
09:34	These two guys want -1 times eight is -8 .
09:39	Okay , so 170 -8 is 162 . Are you
09:46	going so far ? Let's do this last one .
09:48	So this one's five above . This one's five above
09:53	. We add these guys together . So it's 15
09:56	plus 5 , 20 becomes 200 five . Times 5
10:00	is 25 . We get the answer of 225 .
10:06	You're going to see that ? Yeah , this is
10:09	all obviously on paper , but these are not so
10:10	bad to also start doing in your head . Okay
10:13	. The method that the way in which you might
10:15	do , it might vary depending on how comfortable you
10:17	are . Okay . Sometimes you might , you might
10:20	think , well , I'm going to work out this
10:21	time it first so you can keep the eight in
10:23	your head , you're going to take it off the
10:25	answer and then you work out , you know ,
10:26	it's 1 70 take eight . Okay ? Then you
10:28	get 162 . That's all up to you with this
10:31	one . Okay . Anyway , the next video we're
10:33	gonna be having a look at is going to start
10:34	looking at this base 20 idea . So I hope
10:37	to see you then . Bye .

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