GMAT Quantitative Review
3.0 Math Review
3.3 Geometry
7. Quadrilaterals
A polygon with four sides is a quadrilateral. A quadrilateral in which both pairs of opposite sides are parallel is a parallelogram. The opposite sides of a parallelogram also have equal length.
In parallelogram JKLM, 
and 
; 
and 
.
The diagonals of a parallelogram bisect each other (that is, 
and 
).
The area of a parallelogram is equal to
.
The area of JKLM is equal to 
.
A parallelogram with right angles is a rectangle, and a rectangle with all sides of equal length is a square.
The perimeter of 
and the area of WXYZ is equal to 
.
The diagonals of a rectangle are equal; therefore 
.
A quadrilateral with two sides that are parallel, as shown above, is a trapezoid. The area of trapezoid PQRS may be calculated as follows:
.