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simple problem

K. Seshadri jumaseshadri at bmb.leeds.ac.uk
Fri May 19 07:00:16 EST 2000

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.

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.

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