Trapezoidal mean value theorem: the line segment connecting the midpoints of the two waists of a trapezoid is called the trapezoid median line, which is parallel to the two bottoms and equal to half the sum of the two bottoms.
As shown in the figure, the quadrilateral ABCD is trapezoidal, and AD//BC, E and F are the midpoints on the sides of AB and CD respectively. Proof: EF//AD, and EF=(AD+BC)/2.