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  • G'day and welcome to the TecMath channel. In this video today we are looking at the fast

  • way of multiplying two

  • two digit numbers together....so let's go!

  • First of all we'll see if you can multiply some numbers together

  • I'll give you a question which is

  • twenty-one

  • times

  • thirty one. I'll give you six seconds to work that out. Go!

  • So time's up.

  • So if you got the answer of 651 you did really really well

  • you got the right answer

  • If you were busily reaching for the calculator but couldn't find it in time bad luck

  • but you shouldn't need a calculator, and I'll show you how to work it out quickly

  • Don't get all despondant if you didn't work it out I'll show you how to work these out

  • So to get the first number of our answer we multiply the first

  • digits in our two numbers here, so 2 x 3

  • is six, and we have the first part of our answer

  • We leave a blank space.

  • To get the last number of our answer

  • we multiply the last digits of our two numbers here So 1 x 1 is 1.

  • To get the middle part of this number here there is a bit of a trick. What we do

  • is multiply

  • this number and this number

  • and this number and this number and we add the two answers together

  • so 1 x 2 is 2 1 x 3 is 3

  • we add 2 and 3 together and get the answer

  • six hundred fifty one

  • Hey, pretty easy 'eh?

  • We'll have a look at another one

  • So, what about 43

  • times 21, a little bit harder this time

  • See how you go. Have you got the answer yet?

  • I'll start working through this one.

  • So again

  • 4 x 2 is 8

  • We leave the blank space

  • 1 x 3 = 3

  • Then what we do is go...

  • remember this....

  • 1 x 4, and we add this to 3 x 2 so

  • 1 x 4 is 4

  • 3 x 2

  • is 6

  • We add these together

  • We get the answer of 10

  • Now this leads to a problem, so this is a little trick for to understand

  • There is only one space here We only have space to fit one digit

  • So the digit it puts in out of this ten

  • is the zero, and for the one

  • We carry this across

  • And we add it on, so

  • The answer we end up getting

  • 1 + 8 is

  • 9

  • 3...nine hundred and three.

  • Very good. I think we can go through one more

  • I'll rub that out and

  • let's have a look one more question

  • I think this time I'll do it in a nice

  • purple colour

  • and I'll do the question here

  • 42

  • times

  • 63

  • I'm going all out, using different colours today, getting excited

  • Too excited, I can't wait to get the answer - let's work it out

  • First number 4 x6 Leave a space.

  • is 24. Leave a space.

  • 2 x 3 is 6

  • And then what we do

  • And I hope not to get this wrong. I had to remake the video earlier

  • as I stuffed up this one

  • Occassionally

  • I get all excited and I make mistakes too.

  • But for YouTube videos we don't make mistake. We always get 100% correct

  • OK

  • OK

  • We multiply

  • These two guys together. 4 x 3 is 12

  • 6 x 2 is 12

  • And 12 + 12 is 24. We add 24 on, OK?

  • We put the 4 there and 2 there.

  • So what answer did we get?

  • Two thousan

  • six hundred

  • and forty six

  • Hey that's the right answer. Pretty cool right?

  • So if you practice these

  • you will get really good at them and really fast. Your teacher will be

  • utterly astounded about how good you are. Everyone will think

  • your a genius alright?

  • I mean don't go too far - people might stop talking to you

  • but hey -

  • you do this, you'll do really well.

  • And yes you may make a few mistakes. We all make them.

  • So finally I will leave you with a few questions

  • I'll write them, get you to pause the video and work through them, then I'll work through

  • the answers as quick as I can. Here they are.

  • 22

  • times 34

  • I mean 32

  • see how you go on that one

  • fifty-one times

  • sixty three

  • and 82

  • times

  • fifty-one

  • pause the video

  • Then I'll get back to you

  • A couple of seconds and we'll go through these.

  • OK so hopefully you've worked through those

  • and we'll go through the answers right now.

  • OK so

  • 2 x 3 is 6

  • We leave the blank space. 2 x 2 is 4 and then we add four

  • and six together

  • which is ten. We get the answer 704

  • The next one

  • 5 x 6

  • is 30

  • Leave a space

  • 1 x 3 is 3

  • and then we get

  • the number 3 x 5 is 15, 1 x 6 is

  • 6

  • 15 plus 6

  • is 21, so I'll put that in a different colour so it fits

  • This will be a one, and this will be a 2 so our answer is

  • three thousand

  • two hundred and thirteen

  • On to the last one

  • 5 x 8

  • is forty

  • Leave a blank space

  • 1 x 2 is 2

  • then we go 1 x 8 and we add that to 10

  • 8 plus 10

  • is 18 - we put the 8 there and the one there.

  • So our answer is four thousand

  • one hundred and eighty two

  • I hope you did well on those. See you next time

G'day and welcome to the TecMath channel. In this video today we are looking at the fast

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A2 初級

數學秘技 (Fast Math Tricks)

  • 262 37
    Bill Yu 發佈於 2013 年 05 月 01 日
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