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A rectangle is a quadrilateral (a four-sided plane figure) whose angles are all right angles. The opposite sides of a rectangle are parallel and equal in length. Each interior angle is a right angle (90°).

Any side can be chosen as the base and the altitude is the length of a perpendicular line segment between the base and the opposite side (Figure 1). A diagonal is either of the line segments joining opposite vertices.

The area (A) of a rectangle with sides a and b is a× b, the perimeter (P) is a + b, and the length of the diagonal is

A rectangle with a = 4 inches and b = 6 inches has an area of: A = 4 inches× 6 inches = 24 square inches. The perimeter of this rectangle is: P = 4 inches + 6 inches = 10 inches.

A square is a special case of a rectangle where all of the four sides are of equal length. A square with side a of 4 inches has an area of: A = 4 inches× 4 inches = 16 square inches. This same square has a perimeter of: P = 4 inches + 4 inches = 8 inches.

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