note: pie=pi=π
divide both sides by 4pi
[tex]\frac{A}{4\pi}=r^2[/tex]
sqrt both sides
[tex]\sqrt{\frac{A}{4\pi}}=r[/tex]
[tex]r=\sqrt{\frac{A}{4\pi}}[/tex]
[tex]r=\frac{\sqrt{A}}{\sqrt{4\pi}}[/tex]
[tex]r=\frac{\sqrt{A}}{2\sqrt{\pi}}[/tex]
we need to rationalize the denomenator
multiply it by (√π)/(√π)
[tex]r=\frac{\sqrt{A\pi}}{2\pi}[/tex]
