Trapezoid
A trapezoid is defined as a shape with four sides, which has one set of parallel sides.
The area of a trapezoid can be computed as the length of the midsegment, multiplied by the distance along a perpendicular line between the parallel sides. This yields as a special case the well-known formula for the area of a triangle, by considering a triangle as a degenerate trapezoid in which one of the parallel sides has shrunk to a point. Therefore, if a and b are the two parallel sides and h is the distance (height) between the parallels, the area formula is as follows:
A = ½(a + b) h
The quantity ½(a + b) is the average of the horizontal lengths of the trapezoid, so the area can be understood to be the product of the average length and height of the shape.
In an isosceles trapezoid (pictured below), the base angles are congruent, and so are the pair of non-parallel opposite sides.
