Math Antics - Multi-Digit Multiplication Pt 1 - Free Educational videos for Students in K-12

Math Antics - Multi-Digit Multiplication Pt 1 - By mathantics

Transcript


00:03	Uh huh . Hi and welcome to Math Antics .
00:08	In this video lesson , we're going to learn how
00:10	to do basic multi digit multiplication . Now this can
00:13	be tricky at first but don't get discouraged . It
00:16	just might take some extra time and practice but you'll
00:19	get the hang of it . One thing I want
00:21	to mention before we start is that multi digit multiplication
00:24	will be much easier for you . If you memorize
00:26	your basic multiplication facts , but if you don't have
00:29	them memorized , then be sure to have a multiplication
00:32	table handy when you're doing your practice problems . Okay
00:36	. The best way to learn this is to jump
00:37	right in . Let's try 137 times five . Now
00:42	as you can see this is a three digit number
00:44	times of one digit number . When you're given a
00:47	problem like this where the numbers are written side by
00:49	side , you need to rewrite it and stacked form
00:52	with the ones places lined up the way we did
00:55	with addition and subtraction . Now you might remember with
00:58	multiplication that the order of the problem doesn't matter .
01:01	So we can put whatever number we want on the
01:03	top or the bottom , but it's always best to
01:06	put the number with the most digits on the top
01:09	and the one with the fewest digits on the bottom
01:11	there . Now we draw our answer line below and
01:14	we put our time symbol to the left . So
01:16	we don't forget that we're multiplying . Okay , now
01:20	we're ready to learn the procedure for multiplying basically what
01:23	we're gonna do is break up this big multiplication problem
01:26	into a series of small multiplication problems or steps .
01:30	Those small steps involved multiplying two digit in the bottom
01:33	number by a digit in the top number . Now
01:36	in our problem we only have one digit on the
01:38	bottom and we're going to multiply it by each of
01:41	the digits in the top number . And since the
01:43	top number has three digits , that means that our
01:46	problem will have three small multiplication steps . For the
01:50	first step we multiply the bottom digit five by the
01:53	digit in the ones place of the top number which
01:55	is seven . We always start with the ones place
01:58	and work our way from right to left . So
02:00	five times seven gives us 35 . Now , just
02:04	like with addition when we get a two digit answer
02:06	, we have to carry the first digit to the
02:08	top of the next column because it will be in
02:10	the way of the next answer digit if we don't
02:13	. So the five stays down here in the ones
02:15	place of the answer . But the three gets carried
02:17	up to the top of the tens . Place calm
02:20	. Now it's time for the second step we multiply
02:22	the bottom digit five by the next digit to the
02:25	left . And our top number three . So five
02:28	times three equals 15 . But wait a second .
02:32	There's that three we carried up to the top from
02:34	the last answer . What do we do with that
02:36	? Well we need to add it to the answer
02:38	that we just got . That's because that three was
02:41	supposed to go in the tens place of the answer
02:43	but just not all by itself . We had to
02:45	do the second multiplication step first and see what else
02:49	needed to go in that answer spot with it .
02:51	And it turned out to be a five , the
02:53	second digit of the answer 15 . So we have
02:55	to combine the two digits that both go in that
02:58	answer place . That's why we add the digit we
03:00	carried and we end up with 18 . But once
03:04	again our answer is a two digit number . So
03:06	we leave the eight in our answer line and we
03:09	carry the one up to the top of the hundreds
03:11	place column . Now for our third and last multiplication
03:14	step we have five times one which is just five
03:18	. But again there's that digit we carried up above
03:20	so it wouldn't be in the way . What do
03:22	you think we do with that digit , yep ,
03:24	we added to the answer we got from the multiplication
03:27	step , so five plus one gives a six and
03:30	that's what goes in our answer line . Okay then
03:33	we've done all three of our multiplication steps and the
03:36	answer to our problem five times 137 is 685 .
03:41	Now that wasn't so bad , was it ? Let's
03:44	try one more example before you practice some on your
03:46	own . Let's multiply 2617 by four . Again we
03:52	stack our problem with the ones places lined up and
03:55	we make sure that the number with the most digits
03:57	is on top . Because our top number has four
04:00	digits this time it means we'll break this problem up
04:03	into four small multiplication steps will multiply the bottom digit
04:07	by each digit in the top number , starting with
04:10	the ones place on the right and working our way
04:12	left . Okay here we go with the first step
04:15	we multiply the bottom digit by the top digit in
04:18	the ones place four times seven equals 28 . Because
04:22	we have a two digit answer , we need to
04:23	carry the digit that will be in the way we're
04:25	going to add it back in after we complete the
04:27	next multiplication step . So we carry the to to
04:30	the top of the 10s column and we leave the
04:32	eight where it is Now we can do the second
04:35	step . We multiply the bottom digit by the next
04:37	digit in the top number four times one equals four
04:41	. That was easy . But don't forget about that
04:43	digit we carried . We need to add that to
04:45	our answer from the second step . So four plus
04:48	two gives us six and that's what we put in
04:51	our answer place and this time we have one digit
04:54	answer so we don't need to carry . We can
04:56	just move on to the next step For our third
04:59	multiplication step will multiply four times six and that gives
05:02	us 24 , yep , it's a two digit answer
05:05	. So we'll need to carry again . The four
05:07	stays in the answer and the two moves up to
05:10	the top of the thousands , calm Now it's time
05:13	for the 4th and last step we multiply four times
05:16	two which is eight but then we need to add
05:19	the digit we carried , which is a two .
05:21	So eight plus two gives us 10 . And that
05:24	can go down in our answer line even though it's
05:26	a two digit number we don't have to carry because
05:29	we're all done with our multiplication steps and we have
05:31	our final answer four times 2617 equals 10468 . All
05:40	right . That's the basics of multi digit multiplication .
05:43	But it's really important to practice so you get good
05:45	at it . Be sure to do the printable exercises
05:47	for this section . Then once you think you have
05:50	it down , you'll be ready to move on to
05:52	the next video and learn how to do multiplication .
05:54	When both of the numbers have multiple digits . Thanks
05:57	for watching Math Antics and I'll see you next time
06:00	. Learn more at Math Antics dot com .

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