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