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Coordinates of a point outside a plane

  1. Jun 2, 2012 #1

    If you can get me a hint for solving this matter it would be much appreciated.

    I have the 3D coordinates of three points on a plane A, B, C.

    There's another point G and we know AG, BG, CG.

    My problem is to find the coordinates of point G:cry:

    Thanks in advance!
  2. jcsd
  3. Jun 2, 2012 #2


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    G=(A+AG,B+BG,C+CG) unless I don't understand what you are saying.
  4. Jun 2, 2012 #3
    If you are given:


    And you want to find G=(G_1,G_2,G_3), then knowing:

    1) AG=|A-G|=((A_1-G_1)^2+(A_2-G_2)^2+(A_3-G_3)^2)^(1/2)
    2) BG=|B-G|
    3) CG=|C-G|

    is not enough information.

    Let h be the perpendicular distance from G to the plane. Then if A, B, and C and there is another point G' on the other side of the plane with perpendicular distance h from the plane also satisfynig 1, 2 and 3 so the solution is not unique. If A, B, and C are collinear you can find an entire circle of points satisfying 1, 2 and 3.

    If you know that the A, B, and C are not collinear and which side of the plane G is on, then you can solve for the components of G using the distance formula or trigonometry.
  5. Jun 3, 2012 #4
    Thanks mathman and sin(1/x) for your input :shy:.

    I've made a drawing with the problem. I hope all the data are there.

    Sorry for my omissions.

    Attached Files:

  6. Jun 3, 2012 #5


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    I get it now. G=A + |AG|a = B + |BG|b = C + |CG|c, where a,b,c are (unknown) unit vectors. By eliminating G, you will have 6 linear equations for the coordinates of a,b, and c. Using the fact they are unit vectors gives 3 quadratic equations. Solving for them will give you the vectors a,b,c. There will be 2 true solutions. When you solve the quadratics there will be extra solutions, so you need to check to see if you get the same value for G from a given set a,b,c.
    Last edited: Jun 3, 2012
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