Question 636895
Find the radius of a circle with these three points: (1,-7) , (-1,-9), and (3,-9) and center (1,-9)
<pre>
We draw those points:

{{{drawing(3600/13,400,-4,5,-11,2, graph(3600/13,400,-4,5,-11,2),
circle(1,-7,.1), circle(3,-9,.1),circle(-1,-9,.1), locate(1,-9,center),
circle(3,-9,.1),circle(1,-9,.1) )}}}
(1,-7) , (-1,-9), and (3,-9) and center (1,-9)
We can only draw this circle with that center and going through
those points:

{{{drawing(3600/13,400,-4,5,-11,2, graph(3600/13,400,-4,5,-11,2),
circle(1,-7,.1), circle(1,-9,.1), circle(3,-9,.1),circle(-1,-9,.1),locate(1,-9,center),circle(1,-9,2) )}}}

So draw in three radii:

{{{drawing(3600/13,400,-4,5,-11,2, graph(3600/13,400,-4,5,-11,2),
circle(1,-7,.1), circle(-1,-9,.1), circle(1,-9,.1), locate(1,-9,center),circle(1,-9,2),
green(line(1,-9,3,-9)),circle(3,-9,.1), red(line(1,-7,1,-9)),blue(line(1,-9,1,-11)) )}}}

Look at the red radius. it goes from -7 down to -9, that's 2 units
Look at the green radius, it's also 2 units long, and the blue one
is also 2.  So the radius is 2. 

Edwin</pre>