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HomertheGenius
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