Derivation process of surface area formula of cone frustum (talk about how to add each surface area)

suppose that the radius of the bottom surface of the frustum is r, the radius of the top surface is r, and the height of the frustum is h; Then the assumed volume of large cone V1 = 1/3 * π * H1 * r; The volume of the small cone V2 = 1/3 * π * H2 * R 2, obviously R: R = H2: H1;

then the volume of the frustum of a cone V = 1/3 * π *(h1*R*R-h2*r*r)

substitute r=R * h2 /h1 into the above formula v = 1/3 * π * ((H1 3-H2 3)/H1 2) * R. /h1 2) * r 2

= 1/3 * π * h * (1+H2/h1+H2 2/h1 2) * r 2

and then substitute R * h2 /h1 =r into the above formula v = 1/3 * π * h (r