Chapter 9 Quadratic Relations and Conic Sections - 9.6 Translate and Classify Conic Sections - 9.6 Exercises - Problem Solving - Page 656: 48

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The standard form for a circle is: $(x-h)^2+(y-k)^2=r^2$ The circles are 8 ft in diameter, so the radius is $r=4$ The foci: $(h_1,k_1)=(0,4)\\(h_2,k_2)=(0,-4)$ Plug in the given values: $$(x-0)^2+(y-4)^2=4^2\\x^2+(y-4)^2=16$$ $$(x-0)^2+(y+4)^2=4^2\\x^2+(y+4)^2=16$$