How to convert a polar equation of a circle to rectangular

The video tutorial explores a mathematical problem involving multiplication and the use of R-squared. Initially, the teacher considers multiplying by cosine but decides against it. Instead, they explore squaring both sides of an equation to simplify it, leading to the realization that R-squared equals 9. This is interpreted as a circle with a radius of three centered at the origin. The teacher encourages students to verify this result using a calculator.

How is the equation R-squared = 9 related to the geometry of a circle?

What would you observe if you typed the equation into a calculator?