> Can someone tell me how to find the center point of a circle
> when given the radius and two points which lie on the circle.

Recall that two points and a radius are defined by two
circles -- mirror images -- on either side of the chord
line connecting the two given points.  Further data is
needed to decide which circle is the desired one.


method 1
--------

1) set a compass (not magnetic, point & pencil type) to the
   exact given radius of the circle

2) put the compass' pivot on either given point, draw a half
   circle arc on the side towards the other given point

3) put the compass' pivot on the other given point, draw an
   arc on the side towards step 2's arc -- two intersections
   are the centers of the two possible desired circles.


method 2
--------

1) draw a straight line through the two points

2) set a compass (not magnetic, point & pencil type) to a
   reasonable distance, usually slightly larger than the
   chord generated in step 1

3) put the pivot on either given point, draw a half circle
   arc on the side towards the other point

4) put the compass' pivot on the other given point, draw a
   second half circle arc on side towards step 3's arc

5) the arcs in steps 3 & 4 intersect in two places; use the
   straight edge to draw a line through these two intersection
   points -- this line bisects the chord (which may be handy)

6) set the compass for the exact radius of the circle

7) put the compass' pivot on either given point, draw an arc
   which intersects the chord bisector line -- intersection
   is the center of the desired circle.


Or use both methods together -- skew between the "answers"
gives you a measurement of your error in finding the center.

                                                Lee Jones

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