Estimating and Costing
The assumption on which the trapezoidal formula for volumes is based, is

The volume of the Prismoidal is over-estimated and hence a Prismoidal correction is applied
All listed here
The end sections are parallel planes
The mid-area of a pyramid is half the average area of the ends

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Estimating and Costing
In the mid-section formula

The area of mid-sections is calculated by using mean depth
The mean depth is the average of depths of two consecutive sections
All of these
The volume of the earth work is calculated by multiplying the mid-section area by the distance between the two original sections

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