What is the equation of a circle whose center is at the origin and whose radius is 9? x2 y2 = 3 x2 y2 = 9 x2 y2 = 81?

The equation of a circle:

(x - p) ² + (y - q) ² = r²

Coordinates of the center are p and q. The equation of a circle whose center is at the origin (p = 0, q = 0):

x² + y² = r²

Since the radius is 9 (r = 9):

x² + y² = 9²

Answer: x² + y² = 81