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'Two Points Plus One Line' printed from http://nrich.maths.org/
Draw a line (considered endless in both directions), put a
point somewhere on each side of the line. Label these points $A$
Use a geometric construction to locate a point, $P$, on the
line, which is equidistant from $A$ and $B$.
Can this point $P$ always be found for any position of $A$ or
If you believe that this is true how might you construct a
If it is false, identify the circumstances when no point $P$,
equidistant from $A$ and $B$, exists.
Draw a new arrangement of one line and two points, one on each
side of the line.
Imagine creating a collection of similar arrangements.
Can you suggest useful parameters which would uniquely define
or identify each arrangement?
Parameters are the numbers (measurements or ratios) you might
communicate to another person, say over the telephone, if you
wanted them to produce exactly the same arrangement as your
These parameters define each arrangement.
Mark a point $O$ somewhere on the line and express the length
$OP$ in terms of your parameters.