Question 929900
<pre><font face = "Tohoma" size = 3 color = "indigo"><b> 
Hi,
*[illustration circle_2]
A = {{{pi*r^2}}}
let x be the width of the border
{{{(pi*6^2)/2 -pi*(6-x)^2 = 0}}}
{{{(pi*6^2)/2 = pi*(6-x)^2 }}}
 18 = (6-x)^2 = 36 - 12x +x^2
x^2  - 12x + 18 = 0
x = 1.75735931288072
 *[invoke quadratic "x", 1, -12,18]