Question

What is the general form of the equation of a circle with a center at the origin and a radius of 9?

Answer 1

`x^2+y^2=81`

Explanation:

A circle of radius `r` centered at a point `(x_0, y_0)` has the equation

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

Substituting `r=9` and the origin `(0,0)` for `(x_0,y_0)` this gives us

`x^2+y^2=81`