Problem: find the area of trapezium whose parallel sides are 20 cm and 18 cm long , and the distance between them is 30 cm ?
Rationale: "area of a trapezium = 1 / 2 ( sum of parallel sides ) * ( perpendicular distance between them ) = 1 / 2 ( 20 + 18 ) * ( 30 ) = 570 cm 2 answer : b"
annotated_formula: quadrilateral_area(30, 18, 20)