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Homework Help: Dot Product

  1. Mar 5, 2008 #1
    Three vectors x, y and z satisfy x + y + z = 0. Calculate the value of
    (x dot y) + (x dot z) + (y dot z) if the magnitude of x is 2, the magnitude of y is 3, and the magnitude of z is 4.

    Sorry I don't have time to format this properly, I've got to run off to work. This question has been bugging me, and any help would be greatly appreciated.
     
  2. jcsd
  3. Mar 5, 2008 #2

    arildno

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    Dot your equation with x, y and z, and add the results and see something fun happening.
     
  4. Mar 5, 2008 #3
    Sorry, but I'm not sure I follow.
     
  5. Mar 5, 2008 #4

    tiny-tim

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    Do a bit at a time!

    Hi Fineline! Welcome to PF! :smile:

    (btw, it's ok to write x.y + x.z + y.z, once you've made it clear that x y and z are vectors.)

    Yes, I don't follow either: x.y etc is a scalar, so you can't dot it with anything. :frown:

    Hint: Can you calculate x.y + x.z? And then … :smile:
     
  6. Mar 5, 2008 #5
    I do not have any experience with these problems, so I don't know the standard way to approach it. One possible approach, however, is to find what (x + y + z)2 is when expanded.
     
  7. Mar 5, 2008 #6

    tiny-tim

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    ooo! Your method is even nicer than mine! :blushing:
     
  8. Mar 5, 2008 #7

    Hurkyl

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    You have an arithmetic operation. (dot product) Use it. In as many ways as you can imagine.
     
  9. Mar 5, 2008 #8
    Alright, thanks everyone, I got it all worked out.
     
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