## Calculus: Early Transcendentals 8th Edition

$x^2+y^2+z^2 = r^2$ is our given formula for a sphere. Adjusting this to $x^2+y^2+z^2 \leq r^2$ tells us we are only working with points contained on or within the sphere. When $r^2 = 4$, $r= 2$.