Volume of a trapezium = height * (top area +botton area)/2
Assuming that by "trapezium" you mean "a three dimensional figure having parallel top and bottom and planar sides", yes. Basically, '(top area+ bottom area)/2' is the "average" base area. As long as you have linear formulas (and since you have planar sides, this is linear) you can calculate an average like that.
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