Square
In a square: (1) every side is equal (2) the opposing sides are parallel (3) all the angles are 90°. The area, A, of a square can be found using the equation A = a2, where a is the length of a side of the square. Notice that the equation for the area of a square is a specific version of the rectangle equation below.
Rectangle
In a rectangle: (1) opposing sides of a rectangle are equal and parallel (2) every side is not necessarily equal in length (3) all of the angles are 90°. The area, A, of a rectangle can be found using the equation A = l × w. In this case, l stands for the length and w stands for the width. It does not matter which sides are labeled the length or the width.
Parallelogram
In a parallelogram, opposing sides are parallel and equal in length. Opposing angles are equal but not necessarily 90°. The area, A, of a parallelogram can be found using the equation A = b*h. In this case, b stands for the base of the parallelogram and h stands for the height. Any side can be chosen to be the base, but the height is the line that is perpendicular to the base and opposing line.
Trapezoid
In a trapezoid, one set of opposing sides is parallel, but not necessarily equal. The area, A, of a trapezoid can be found using the equation A = .5 × (a + c) × h. In this case, h stands for the line that is perpendicular to parallel sides a and c.
Rhombus
In a rhombus: (1) all sides are equal in length (2) opposite sides are parallel (3) diagonals bisect each other (4) the intersection of diagonals forms a 90° angle.