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Formula y=kx+n help

  1. Mar 28, 2008 #1
    1. The problem statement, all variables and given/known data

    Can somebody use the formula y=kx+n, to present me formula for the line which is parallel to the y axis and go among the point (-4,-5)?

    2. Relevant equations

    y=kx+n

    3. The attempt at a solution

    k=tg(alpha)

    k=tg90

    k=[tex]\frac{1}{0}[/tex]

    y=[tex]\frac{x}{0}[/tex]

    x=0

    -4=0

    Hm.....
     
  2. jcsd
  3. Mar 28, 2008 #2
    If the line is parallel to the Y AXIS then the slope has to be undefined, see why?
     
    Last edited: Mar 28, 2008
  4. Mar 28, 2008 #3

    rock.freak667

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    you can't multiply by zero like that.

    The y-axis is the vertical one,right?
     
  5. Mar 28, 2008 #4
    wouldn't that be just x=-4?
     
  6. Mar 28, 2008 #5

    rock.freak667

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    That's what I believe it will be.
     
  7. Mar 29, 2008 #6
    I know that it will be x=-4. But please prove it with y=kx+n
    rock.freak667, y is the vertical one.
     
  8. Mar 29, 2008 #7
    The problem is that the two assumptions

    k, and n exist such that {(x,y): y=kx+n} is the set of points of the line

    {(x,y): x = -4} is the set of points of the line

    can not both be true.
     
  9. Mar 29, 2008 #8
    what is k, what is n?
     
  10. Mar 29, 2008 #9

    HallsofIvy

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    You can't. A vertical line, parallel to the y-axis, cannot be written in that form.
     
  11. Mar 29, 2008 #10
    Why?
     
  12. Mar 29, 2008 #11

    HallsofIvy

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    For exactly the reason everyone has been telling you! Writing "y= kx+ n" means that different values of x give different values of y. That is true for all lines except vertical lines. A line parallel to the y-axis has the same value of y for every x. When you learned "y= kx+n", the "slope-intercept" form, you should have learned that every non-vertical line can be written in that form but vertical lines cannot.
     
    Last edited: Mar 29, 2008
  13. Mar 29, 2008 #12
    Ahhh... I undertand now. Thank you very much.
    btw- what is that same value for y?
     
  14. Mar 29, 2008 #13

    HallsofIvy

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    My mistake. I meant to say the "same x value for every y". In this case, since you tell us one point is (-4, -5) that x value is x= -4 for all y. That's why we can write the equation of that line "x= -4" without mentioning y.

    (A horizontal line would have the "same y value for every x". The horizontal line (parallel to the x-axis) passing through (-4, -5) would have y= -5 for every y. Of course, that is of the form "y= kx+ n" with k= 0, n= -5.)
     
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