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Homework Help: A plane through three points

  1. Jul 27, 2016 #1

    I'm currently reading Calc III by Marsden & Weinstein. One of the examples shows a plane being drawn through three points. While I understand their solutiom, I'm very curious as to why my solutiom doesn't work.

    1. The problem statement, all variables and given/known data

    Write the equatiom for a plane through A = (1, 1, 1), B = (2, 0, 0) and C = (1, 1, 0).

    2. Relevant equations
    For a plane through Point P = (Px, Py, Pz) orthogonal to n = <A, B, C> we write the equation:

    A(X - Px) + B(Y - Py) + C(Z - Pz)

    3. The attempt at a solution
    The book solves the problem by filling the points into the more general formula:

    Ax + By + Cz + D = 0

    Which leaves us with three equations and four unknowns, which is adequate to come up with a solution (there is an infinite amount of solutions).

    I tried to solve it by trying to find a normal vector n = <nx, ny, nz> that is orthogonal to the vectors AB, AC and BC.

    n ⋅ AB = 0
    n ⋅ AC = 0
    n ⋅ BC = 0

    Leads to

    nx - ny - nz = 0
    -nz = 0
    -nx + ny = 0

    Solving this system leads to twice the expression nx = ny and once nz = 0. Thus I took <1, 1, 0> as a possible n.

    Filling this in alongside A into the relevant equation leads to the equation

    (x - 1) + (y - 1) = 0

    Which simplifies to

    x + y = 2

    Obviously this is not a solution and the fact that the final answer incorporates a nomzero coeficient for z only proves that point. What did I do wrong?

    - HF
  2. jcsd
  3. Jul 27, 2016 #2

    Stephen Tashi

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    Science Advisor

    What final answer are you referring to ? The final answer given in the book ?
  4. Jul 28, 2016 #3


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    Homework Helper

    Why do you think your solution is wrong? Do coordinates of A, B, C fulfill the equation x+y=2?
  5. Jul 28, 2016 #4

    Stephen Tashi

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    Science Advisor

    Have you've visualized that the answer to the problem is a plane that is perpendicular to the xy-plane, so it "sticks straight up in the z-direction"? (The answer must be a plane containing the vertical line segment (1,1,0) to (1,1,1). ) The lack of the "z" variable in "x + y = 2" is what permits the z-coordinate to take arbitrary values.
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