A trapezium is a quadrilateral with one pair of opposite sides
parallel.

The ends of a garden shed or the sides of a swimming pool
are often trapezia.

The area of a trapezium is given 1/2 (a + b) h, where the
parallel sides are of lengths a and b, and h is the distance
between them. It can be found by dividing the trapezium into
two triangles by a diagonal as shown whose areas are 1/2 ah
and 1/2 bh.