On Jan 7, 8:48 am, Sly Pandemia <slypande...@gmail.com> wrote:
> On Fri, 6 Jan 2012 14:41:10 -0800 (PST), "seto...@att.net"<seto...@att.net> wrote:
> 
> > If you choose e1 as origin then '20m right of e1 and 20s later of e1'
> > will have meaning'....the coordinate of e3 with e1 as origin is (+20m,
> > +20s).
> 
> Okay, fine. Let's choose e1 as the origin.
> e3 is 20m to the right of e1 and 20s later than e1. You correctly
> state that "the coordinate of e3 with e1 as origin is (+20m,+20s)"
> 
> So what are the coordinates of e4, which is 20m to the left of e1 and
> 20s earlier?

The coordinate of e4 with e1 as origin is [deta(-20m),delta(-20s)]