Question

What is the equation of a circle whose center is at the origin and whose radius is 9?

Answer 1

The standard equation for a circle is \[(x - h)^{2} + (y - k)^{2} = r ^{2}\]
with (h,k) as your center and r being the radius.

Answer 2

a) x^2 + y^2 = 3
b) x^2 + y^2 = 9
c) x^2 + y^2 = 81