Calculus: Early Transcendentals 8th Edition

The equation of a circular cylinder with radius $2$ and axis the z-axis.
Given: $r=2$ As we know that $r^2=x^2+y^2 \implies r=\sqrt{x^2+y^2}$ Thus, we have $\sqrt{x^2+y^2}=2$ This can be written as: $x^2+y^2=2^2$ Thus, it represents an equation of a circular cylinder with radius $2$ and axis the z-axis.