## Calculus: Early Transcendentals 8th Edition

Sphere with radius $2$ and centre at the origin.
Given: $r^2+z^2=4$ As we know that $r^2=x^2+y^2 \implies r=\sqrt{x^2+y^2}$ Thus, we have $x^2+y^2+z^2=4$ This can be written as: $x^2+y^2+z^2=2^2$ Thus, it represents an equation of a sphere with radius $2$ and centre at the origin.