Question

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

Answer 1

x^2+y^2 = 9^2

Answer 2

\(\normalsize\color{blue}{ (x-h)^2+ (y-k)^2=r^2}\)

(h,k) is the center
r is the radius

\(\normalsize\color{blue}{ (x-\color{red}{0} )^2+ (y-\color{red}{0})^2=\color{red}{9}^2}\)

\(\normalsize\color{blue}{ x^2+ y^2=81}\)