# Chapter 7 - Section 7.3 - Simplifying Radicals, the Distance Formula, and Circles - 7.3 Exercises: 134

$x^2+y^2=81$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$ Use the Center-Radius Form of the equation of circles to get the equation with the given center, $C( 0,0 ),$ and the given radius, $r= 9 .$ $\bf{\text{Solution Details:}}$ With the given center, then $h= 0$ and $k= 0 .$ Using the Center-Radius Form of the equation of circles which is given by $(x-h)^2+(y-k)^2=r^2,$ the equation of the circle is \begin{array}{l}\require{cancel} (x-0)^2+(y-0)^2=(9)^2 \\\\ x^2+y^2=81 .\end{array}

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