\[dR, \; R d \phi , \; dz\]
at right angles, so the volume of the cuboid \[dV \simeq R dR d \phi dz\]
.The approximation becomes better as
\[dR, \; d \phi \; dz \rightarrow 0\]
.We can also approximate an element of surface area as the area of a rectangle of base and height
\[R d \phi ,dz\]
respectively so \[dA \simeq R d \phi dz\]
.Again the approximation improves as
\[ d \phi \; dz \rightarrow 0\]
.