The side surface of the frustum is developed into a fan-shaped ring, and its surface area is equal to the area of the upper and lower bottom surfaces plus the area of the side surface.

Area π r 2 of upper and lower surfaces

The area of underground π r' 2

Transverse area ∏(r'L+rl)

Platform volume =1/3π (R2+R2+RR) √ [L2-(r-r) 2]

I didn't bring a book during the summer vacation, so I will give you the answer by memory.

But the idea is right, but it is not very detailed.