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Homework Help: I really need help as soon as possible

  1. Oct 27, 2013 #1

    I really need help. All the information is on that link.

    I've spent 30 minutes or so trying to figure this out but I can't and I'm starting to get really upset.

    If you guys can find the value of k, I can take it from there. I just have no idea how to find k and it's stressing me out so much.
  2. jcsd
  3. Oct 27, 2013 #2

    Simon Bridge

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    Welcome to PF;
    ... is there a special meaning for the capitalization?
    Assuming none:

    Consider the two planes that do not have k in them - they must intersect along a line right?
    What does the third plane have to avoid doing to make all three intersect at a point?
  4. Oct 27, 2013 #3
    No, no meaning for caps.

    I actually did consider them. My problem is, what do I do with that idea? How do I solve k based on that? Like I've tried everything and I'm just about to break because I need to know this. My teacher has never given us a question like this so I feel rather hopeless as much as I love this course.
  5. Oct 27, 2013 #4
    Oh and all three planes when using n1 x n2 • n3 must equal 0 and be coplanar. If it does not equal 0, then it intersects at a single point. Again, I just don't know how to find K. I've tried dot product, linear equations, etc
  6. Oct 27, 2013 #5

    Simon Bridge

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    ... think geometry not algebra!
    Start the algebra once you know what you are aiming for.

    In general, three planes will have an intersection at a point. i.e. there is one point that is on all three planes.
    But they don't have to. They actually don't have to intersect at all.

    Two planes will intersect on a line - with three planes there will be 2 or three lines of intersection (where one plane intersects with one of the others).

    In your problem you have two planes that intersect (in a line) so you can't do anything about that.

    You have a third plane whose orientation you get to change by changing the variable k.

    You have to use your imagination - or get some bits of stiff card and hold them up at different angles.

    [edit: and try not to get impatient - I'm typing as fast as I can :( ]
  7. Oct 27, 2013 #6

    Only problem is, I've only been taught to solve with algebra. It tried putting x=0 and solve for x, z = 0 and solve for z, then sub in x and z into equation 2 and try to solve for k, but it's no good!
  8. Oct 27, 2013 #7

    Simon Bridge

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    Do you not know what a plane looks like?
    Can you not imagine a couple of planes in space?
    If you do not, and cannot, I cannot help you.

    You have to know what the equations mean!
  9. Oct 28, 2013 #8
    I figured it out. K=4

    Thanks for your help, I guess..
  10. Oct 28, 2013 #9
    It looks like all she cares about is the answer. It does not matter what this means. :smile:
  11. Oct 28, 2013 #10

    Simon Bridge

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    That's great!
    Do you know why k=4?

    I never even got that far because you indicated you didn't want to go there.

    You understand that we cannot do your homework for you?
    Instead we have to sort-of nudge you onto the right track.

    Good luck.
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