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Math Help - Simple problem... help please.

  1. #1
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    Simple problem... help please.

    Hi,

    I'm sure this is really simple but my brain doesn't seem to want to work and it is around 20 years since I tried something like this.

    I have a line, Line A. I know the x and y coordinates of its start and end points and its length. I also know its angle with the positive x-axis.

    I need to create line B. Line B must be perpendicular to line A and must start at either the start or end point of line A. I will know the length of line B.

    So, given that I know the length, start point, and gradient of line B (negative reciprocal of line As gradient), is there some magic formula that I can feed these numbers in to in order to find the x,y coordinates of the end point of line B?

    I was hoping there was some simple formula that I could just feed the values I know of line B into and get its end coordinates.

    Any help would be greatly appreciated.

    Chicka.
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  2. #2
    Bar0n janvdl's Avatar
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    Try the distance formula. [LaTeX ERROR: Convert failed]
    Last edited by janvdl; October 31st 2007 at 04:39 AM. Reason: Mistake
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  3. #3
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    earboth's Avatar
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    Quote Originally Posted by Chicka View Post
    Hi,

    I'm sure this is really simple but my brain doesn't seem to want to work and it is around 20 years since I tried something like this.

    I have a line, Line A. I know the x and y coordinates of its start and end points and its length. I also know its angle with the positive x-axis.

    I need to create line B. Line B must be perpendicular to line A and must start at either the start or end point of line A. I will know the length of line B.

    So, given that I know the length, start point, and gradient of line B (negative reciprocal of line As gradient), is there some magic formula that I can feed these numbers in to in order to find the x,y coordinates of the end point of line B?

    I was hoping there was some simple formula that I could just feed the values I know of line B into and get its end coordinates.

    Any help would be greatly appreciated.

    Chicka.
    Hello,

    I'll put some flesh to the more or less bony hint which janvdl gave to you:

    Let the equation of line B be: y = mx + b
    and let the endpoint of B be: E(x_E, y_E) that means E(x_E, mx_E + b)

    Now use the distance formula (without the square root) and the point A from line A:

    d^2 = (length)^2=(x_A-x_E)^2+(y_A - (mx_E + b))^2

    That is an equation with only x_E unknown.
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  4. #4
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    Thanks for the help so far, I'll see what I can do with it.
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