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Perpendicular lines

  1. Jan 25, 2010 #1
    A very simple and noob question, sorry for this.

    Looking at http://en.wikipedia.org/wiki/Perpendicular I found that two lines are perpendicular if and only if the product of their slopes is -1.

    If I have two lines described by the following equations:

    y = ax + b
    g = cx + d

    then they're perpendicular if ac = -1 right? Okay...

    What's the explanation for this? How do I conclude that they're only perpendicular if and only if the product of their slopes is -1?

    Just another little question: the slope of a line is the tangent of the angle between the line and the x-axis? that's right?

    Thank you
     
  2. jcsd
  3. Jan 26, 2010 #2

    tiny-tim

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    HI Taturana! :smile:
    That's right :smile: … slope = tan, and so …

    tan(90º + θ) = sin(90º + θ)/cos(90º + θ) = cosθ/(-sinθ) = -1/tanθ. :wink:

    (or you can prove it just as easily by drawing it)
     
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