Question 963976
Imagine drawing a circle on a cartesian system, centered at the origin, and sketch a rectangle whose diagonals will intersect at the center of the circle.  Each of these diagonals is also a diameter of the circle.  You want to find the length value of this diagonal  (either of them, since one will be same size as the other).


Understand that a diagonal cuts the rectangle into two congruent right triangles.  


{{{d=sqrt(5^2+12^2)}}}
{{{d=sqrt(25+144)}}}
{{{d=sqrt(169)}}}
{{{highlight_green(d=13)}}}, the diagonal or the diameter of the circle.


The radius value is {{{highlight(13/2=6&1/2)}}}.