Question

The equation of the circle whose center is (0, 0) and radius is length 9 is x² + y² = 3 x² + y² = 9 x² + y² = 81

Answer 1

The equation of a circle with center
`(x_0,y_0)` and radius
`r` is

`(x-x_0)^2 + (y-y_0)^2 = r^2`

In your case,
`(x_0,y_0) = (0,0)` and
`r=9`

so the equation becomes

`(x-0)^2 + (y-0)^2 = 9^2 \iff x^2+y^2=81`