## Precalculus (6th Edition)

$\color{blue}{x^2+y^2=81}$ Refer to the graph below.
RECALL: The center-radius form of a circle's equation is $(x-h)^2+(y-k)^2=r^2$ with center at $(h, k)$ and a radius of $r$ units. Thus, the given circle's equation in center-radius form is: $(x-0)^2+(y-0)^2=9^2 \\\color{blue}{x^2+y^2=81}$ To graph the circle, perform the following steps: (1) Plot the center. (2) Plot the points 9 units above, below, to the left, and to the right of the center. (3) Connect the four points in Step 2 using a curve to form a circle. (Refer to the attached image in the answer part above for the graph.)