find the area of trapezium whose parallel sides are 28 cm and 20 cm long , and the distance between them is 21 cm ?
"area of a trapezium = 1 / 2 ( sum of parallel sides ) * ( perpendicular distance between them ) = 1 / 2 ( 28 + 20 ) * ( 21 ) = 504 cm 2 answer : c"
quadrilateral_area(21, 20, 28)