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Easy Mental Multiplication Trick

  1. Nov 17, 2006 #1
    Last edited by a moderator: Apr 22, 2017
  2. jcsd
  3. Nov 17, 2006 #2


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    Basically, because of where the tens and the ones intersect, you get one corner as the hundreds (ten*ten), two corners are tens (1*10, 10*1), and one corner is the singles (1*1)

    I only watched the first example, obviously you could expand it to hundreds and thousands and such
  4. Nov 17, 2006 #3
    That's kind of neat.
  5. Nov 17, 2006 #4
    Very nice, I will have to show that to my younger siblings. Thanks for sharing the link.
  6. Nov 17, 2006 #5
    (10 + 2)(20 + 1)
    10 * 20 + 10 * 1 + 2 * 20 * 2 * 1
    Doing the trick does the same thing as multiplying that out the way you would with variables in there. (you know, like if it
    were (x + 1)(x - 5) or something and you wanted to expand it)

    EDIT: That is if this is the trick I'm thinking of.
    Last edited: Nov 18, 2006
  7. Nov 17, 2006 #6


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    moose, that's basically what it does
  8. Nov 18, 2006 #7
    If this method helps you, then go for it. However, it has its limitations. Try this one the traditional way and the graphical way:

    999 X 999
  9. Nov 18, 2006 #8


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    It's an interesting way to multiply numbers, but equivalent to just summing the product of all the digit pairs. Frankly, drawing the picture is just slowing him down. Very creative though, don't think I would have thought of doing it.

    Hey, we should come up with other ways to do math using pictures.
  10. Nov 18, 2006 #9
    I like the trick that you can use to square numbers. Let's say you want to square 28... You know that 30*30 = 900.

    If you then use the derivative of x^2 to approximate it, you subtract 2*30*(30-28), resulting in 780. Then as a correction, you add in the change squared (30-28)^2, so your answer is 784. This is really simple around 50, because you just subtract or add 100s.

    So if you want to find x^2, and know y^2
    x^2 = y^2 - 2y(y - x) + (y - x)^2

    Really fast and easy to do. I would think that most people on this board already know of this "trick" though.
  11. Nov 18, 2006 #10
    Wow, thats pretty neat.

    Here is an even neater trick!

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