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Topic: A simple Solve gives me errors
Replies: 1   Last Post: Apr 24, 2014 4:02 AM

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Murray Eisenberg

Posts: 2,105
Registered: 12/6/04
Re: A simple Solve gives me errors
Posted: Apr 24, 2014 4:02 AM
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In the definitions of eq1 and eq2, ":=", which means SetDelayed, should be "==" (double equals sign), which means Equal and is used to form equations.

On Apr 23, 2014, at 4:54 AM, Ste[hen Gray <stevebg@roadrunner.com> wrote:

> I want to find the center C of a circle in 3D where the plane is defined
> by points p1,p2,p3. I wrote this:
> p1 = {p1x, p1y, p1z}; (* tell it that p1,p2,p3 are vectors *)
> p2 = {p2x, p2y, p2z}; (* center of circle ctr = f p1+g =

p2+(1-f-g)p3 *)
> p3 = {p3x, p3y, p3z}; (* keep the center in the plane of p1,p2,p3 *)
> ctr = f p1 + g p2 + (1 - f - g) p3;
> eq1 = (ctr - p1).(ctr - p1) - (ctr - p2).(ctr - p2) := 0;
> eq2 = (ctr - p1).(ctr - p1) - (ctr - p3).(ctr - p3) := 0;
> I get errors of a type I've seen before:
> SetDelayed::write: Tag Norm in Norm[{-p1x+f p1x+g p2x+(1-f-g) p3x,-p1y+f
> p1y+g p2y+(1-f-g) p3y,-p1z+f p1z+g p2z+(1-f-g) p3z}] is Protected. >>
> And a similar one. I don't know what to do about this.
> I want to do
> Solve[{eq1, eq2}, {f, g}]

Murray Eisenberg murray@math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower phone 240 246-7240 (H)
University of Massachusetts
710 North Pleasant Street
Amherst, MA 01003-9305

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