Respuesta :

altavistard
start with x^2 + y^2 = r^2, where r signifies radius.  Subst. 0 for x and 9 for y:

0^2 + 9^2 = r^2.  Therefore, r = 9.

The std eqn is x^2 + y^2 = 9^2.  this is a special case of 

(x-h)^2 + (y-k)^2 = r^2 and is appropriate because the origin is at (0,0).