Definition: A trapezoid is a quadrilateral with at least one pair of parallel sides, called bases; its area is half the sum of the bases times the height.
What are its parts?
The parallel sides are the bases, and the other two sides are legs. The height is the perpendicular distance between the bases. If the legs are equal, the trapezoid is isosceles, and its base angles are equal.
What is the area formula?
A = (1/2)(b1 + b2)h. Think of it as the average of the bases times the height.
Example. A trapezoid has bases 14 and 22 and height 9. Area = (1/2)(14 + 22)(9) = 162. The midsegment, which joins the midpoints of the legs, has length (14 + 22)/2 = 18 and is parallel to the bases.
What errors occur?
Using a slanted leg as the height, rather than the perpendicular distance. Another error is to demand exactly one pair of parallel sides in a course that defines it with at least one. Adding the bases but forgetting to halve the sum gives twice the correct area. The angles along each leg add up to 180 degrees.