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
x² + y² = 81
 

40 & 50 in the right corner are the complementary angles as when placed in a sum (added together) they make 90°