Answer
The student did not square $r$.
Work Step by Step
We know that the equation for a circle centered at the origin of radius r is $x^2+y^2=r^2$. Thus, the equation for this circle is:
$$x^2+y^2=12^2 \\ x^2+y^2=144$$
Algebra 2 (1st Edition)
by Larson, Ron; Boswell, Laurie; Kanold, Timothy D.; Stiff, Lee
Published by
McDougal Littell
ISBN 10:
0618595414
ISBN 13:
978-0-61859-541-9
Chapter 9 Quadratic Relations and Conic Sections - 9.3 Graph and Write Equations of Circles - 9.3 Exercises - Skill Practice - Page 629: 30
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