Multivariable Calculus, 7th Edition

Published by Brooks Cole
ISBN 10: 0-53849-787-4
ISBN 13: 978-0-53849-787-9

Chapter 12 - Vectors and the Geometry of Space - 12.1 Exercises - Page 814: 17

Answer

The sphere is centered at $(2,0,6)$ and has radius $\frac{9}{\sqrt{2}}.$

Work Step by Step

To show the equation $2x^2+2y^2+2z^2=8x-24z+1$ is an equation of a sphere, we will put it in the form $(x-a)^2+(y-b)^2+(z-c)^2=r^2$, where $a$, $b$, $c$, and $r$ are real numbers. First, we "move" both of the variable terms from the right hand side of the equation to the left hand side, then we divide both sides by two. We get $$x^2-4x+y^2+z^2+12z=\frac{1}{2}.$$ Next we want to complete the squares of $x^2-4x$ and $z^2+12z$, so we add 4 and 36 to both sides: $$x^2-4x+y^2+z^2+12z+4+36=\frac{1}{2}+4+36 \\(x^2-4x+4)+y^2+(z^2+12z+36)=\frac{81}{2} \\(x-2)^2+y^2+(z-6)^2=\frac{81}{2}.$$ Hence, this is a sphere centered at $(2,0,6)$ and with radius $\sqrt{\frac{81}{2}}=\frac{9}{\sqrt{2}}.$
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