Convert $$$r = 9$$$ to rectangular coordinates

Convert $$$r = 9$$$ to rectangular coordinates.

In rectangular coordinates, $$$r = \sqrt{x^{2} + y^{2}}$$$ and $$$\theta = \operatorname{atan}{\left(\frac{y}{x} \right)}$$$.

Thus, the input can be rewritten as $$$\sqrt{x^{2} + y^{2}} = 9$$$.

Simplify: the input now takes the form $$$x^{2} + y^{2} = 81$$$.

$$$r = 9$$$A in rectangular coordinates is $$$x^{2} + y^{2} = 81$$$A.