# simple problem

M. Johan Broekman hanbroekman at compuserve.com
Fri May 19 20:06:13 EST 2000

```"K. Seshadri" wrote:
>
> Mark wrote:
>
> > Given the cartesian coordinates of
> > two points a and b in 3D space, I need the position
> > of a third point c such that the angle c-a-b has a fixed
> > value, say 104.5 degrees.
> >
> >           c
> >            \
> >             a - b
> >
> > how can I obtain the coordinates (x,y,z) of c from the
> > known coordinates of a and b?
> > Clearly there are many equivalent choices of c satisfying
> > this requirement; what I'm looking for is simply the direction of some
> > a-c axis which form the required angle with the a-b axis.
> >
> > is it a trivial problem? I haven't found a solution...
> > any help appreciated
> >
> > Sent via Deja.com http://www.deja.com/
>
> Hi,
> The direction of a-c vector itself is not unique given the angle at a.
> It will sweep a cone about the axiz a-b.   You need to provide atleast
> one more constraint in order to have it fixed.
> -Juma

Juma, Mark wants the equation for a cone at "his" angle to a - b, with
the apex of the cone at a.  There should be an unequivocal equation for
that, but I'm not a good enough math person for that.
--
Best regards
Han Broekman