Question 201095
if the radius of a circle is increase by 4 units, its original area is multiplied by 2. Find the original radius. 
-------------------
{{{A = pi*r^2}}}
{{{2A = 2pi*r^2}}}
{{{2A = pi*(r+4)^2}}}
{{{2r^2 = (r^2 + 8r + 16)}}}
{{{2r^2 = r^2 + 8r + 16}}}
{{{1r^2 - 8r - 16 = 0}}}
*[invoke solve_quadratic_equation 1,-8,-16]
Sub r for x and ignore the negative answer.
r = 4 + 4sqrt(2)
r = ~ 9.65685