Surface area of a frustum of a cone
$$S = \pi\cdot (R+r)\cdot l+\pi\cdot R^{2}+\pi\cdot r^{2}$$
S - surface area
R - radius of lower base
r - radius of upper base
l - slant height
R - radius of lower base
r - radius of upper base
l - slant height
Find
It is known that:
= 