# simple problem

M. Johan Broekman hanbroekman at compuserve.com
Thu May 18 18:56:52 EST 2000

```Mark wrote:
>
> In article <39232CD6.23DE8D21 at compuserve.com>,
>   "M. Johan Broekman" <hanbroekman at compuserve.com> 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/
> > > Before you buy.
> >
> > My very old, and rudimentary trig experiences suggest that there are
> an
> > infinite number of points "c", all laying on a cone shaped surface
> with
> > angle xyz from the line a-b.
> > --
> > Best regards
> > Han Broekman
>
> Of course; what I need is an expression that, starting from the
> direction of the AB axis, provides a generic point xyz
> laying on such cone surface.
> best regards
>                                              mark
>
> Sent via Deja.com http://www.deja.com/

Sorry, Mark, _during_ a math exam I switched from being a chemistry
major to a biochemistry major, because that exam looked easier.  I don't
remember whether I passed, but that was 35 years ago.  Can't help you
now anymore ...
--
Best regards
Han Broekman