## Algebra and Trigonometry 10th Edition

circle $(h,k)$ $r$
We want all the points $(x,y)$ that are equidistant, $r$, from $(h,k)$: $d=\sqrt {(x-h)^2+(y-k)^2}~~$ (Square both sides and make $d=r$: $r^2=(x-h)^2+(y-k)^2$