## Thomas' Calculus 13th Edition

(a) $x^2+y^2=4^2 , z=0$ (b) $y^2+z^2=4,x=0$ (c) $x^2+z^2=4, y=0$
The equations of the circle with radius $r$ and center at origin in the xy-plane is represented as:$x^2+y^2=r^2$ when $z=0$, likewise for $yz-$plane $y^2+z^2=r^2$ when $x=0$ and for $xz-$ plane: $x^2+z^2=r^2$ when $y=0$ Hence, our equations are: (a) $x^2+y^2=4^2 , z=0$ (b) $y^2+z^2=4,x=0$ (c) $x^2+z^2=4, y=0$