Elementary Geometry for College Students (6th Edition)

1. The edges of trapezoids are straight lines. 2. Thus, as a line in between the two sides of the trapezoids goes downward, its length decreases constantly. 3. Because its length decreases constantly, its length will be the average of the upper length and the bottom length. Using the equation for average, we obtain: $EF = 1/2(AB + DC)$