# Circle Equation: Center (0,0)

- Author:
- Tim Brzezinski

## For the questions below, be sure to zoom out if you need to!

## 1.

Suppose *P(x,y)* = any point that lies on a circle with center (0,0) and radius 5.
Use what you've observed to write an equation that expresses the relationship among *x*, *y*, and *r*.

## 2.

What is the equation of a circle with center (0,0) and radius *r* = 9?

## 3.

Suppose another circle has center (0,0). Suppose this circle also passes through the point (12, -5). Write the equation of this circle.

## 4. FINAL QUESTION:

Suppose *P(x,y)* = any point that lies on a circle with center (0,0) and radius *r*, where *r* > 0.
Use what you've observed to write an equation that expresses the relationship among *x*, *y*, and *r*.