Referring to Fig. 10.38, integrate the expression dS = 2πρ2 sin (φ dφ over the lth zone to...

Referring to Fig. 10.38, integrate the expression dS = 2πρ2 sin (φ dφ over the lth zone to get the area of that zone,

Show that the mean distance to the lth zone is

so that the ratio Al/rl is constant