Another service from Omega

The Plane through 3 Points


*****


The equation of a Plane through 3 given points A, B, C.
picture here


> a := vector(3): b := vector(3): c:=vector(3):

The normal vector to the plane will be given by


> n := crossprod(b-a,c-a);

       n := [ (b[2] - a[2]) (c[3] - a[3]) - (b[3] - a[3]) (c[2] - a[2]),

           (b[3] - a[3]) (c[1] - a[1]) - (b[1] - a[1]) (c[3] - a[3]),

           (b[1] - a[1]) (c[2] - a[2]) - (b[2] - a[2]) (c[1] - a[1]) ]



and to compute the equation of the plane we do


> r := vector([x,y,z]):
> innerprod(n, r-a);

   - b[2] a[3] x - a[2] c[3] x + b[2] c[3] x + b[3] a[2] x + a[3] c[2] x

        - b[3] c[2] x + a[1] c[3] y + b[1] a[3] y - a[3] c[1] y - b[1] c[3] y

        + b[3] c[1] y - b[3] a[1] y - b[2] c[1] z + b[1] c[2] z - b[1] a[2] z

        - a[1] c[2] z + b[2] a[1] z + a[2] c[1] z - b[2] c[3] a[1]

        + b[1] c[3] a[2] + b[3] c[2] a[1] + b[2] c[1] a[3] - b[1] c[2] a[3]

        - b[3] c[1] a[2]



In order to "collect" terms in x, y and z we write,

> collect(",[x,y,z]);

(b[2] c[3] - b[2] a[3] - a[2] c[3] - b[3] c[2] + b[3] a[2] + a[3] c[2]) x

    + (b[3] c[1] - b[3] a[1] - a[3] c[1] - b[1] c[3] + b[1] a[3] + a[1] c[3]) y

    + (b[1] c[2] - b[1] a[2] - a[1] c[2] - b[2] c[1] + b[2] a[1] + a[2] c[1]) z

    - b[2] c[3] a[1] - b[1] c[2] a[3] + b[1] c[3] a[2] + b[2] c[1] a[3]

    + b[3] c[2] a[1] - b[3] c[1] a[2]


The equation of the plane is then given by the previous expression = 0. It is more convenient to pack the above commands into a maple procedure as follows:


> P3points := proc(a,b,c)
> local ex1,ex2,n,_x,_y,_z,r;
> n := crossprod(b-a,c-a);
> r := vector([_x,_y,_z]);
> ex1 := innerprod(n,r-a);
> ex2 := collect(ex1,[_x,_y,_z]);
> ex2 = 0
> end;


Let's test it,


> P3points([0,0,-8],[0,4,0],[8,0,0]);

                        32 _x + 64 _y - 32 _z - 256 = 0


OK this is the answer but to show how you can use maple to re-organize the way an equation looks we load the very useful "isolate" command with,

> readlib(isolate):

> isolate(P3points([0,0,-8],[0,4,0],[8,0,0]),- 256);

                         -256 = - 32 _x - 64 _y + 32 _z


Do you see how -256 was isolated on the right? We can also divide the whole equation through by 32 with,


> "/32;


                             -8 = - _x - 2 _y + _z



and now let's flip the rhs and lhs with,


> isolate(",rhs("));


                             - _x - 2 _y + _z = -8




Multiply through by (-1)....


> "*(-1);


                               _x + 2 _y - _z = 8




one more example,


> P3points([1,1,-2],[-1,1,0],[0,2,2]);


                          - 2 _x - 8 + 6 _y - 2 _z = 0



> "/(-8);

                        1/4 _x + 1 - 3/4 _y + 1/4 _z = 0




sorry I just wanted to divide by (-2), but not a big deal... just do


> "*4;


                             _x + 4 - 3 _y + _z = 0



> isolate(",4);

                              4 = - _x + 3 _y - _z




and flip it around...


> isolate(",rhs("));


                              - _x + 3 _y - _z = 4



Link to the commands in this file
Carlos Rodriguez <carlos@math.albany.edu>
Last modified: Wed Oct 23 10:56:54 EDT 1996