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A rectangle is a quadrilateral whose angles are all right angles. The opposite sides of a rectangle are parallel and equal in length. Any side can be chosen as the Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. base and the altitude is the length of a perpendicular line segment between the base and the opposite side (see Figure 1). A diagonal is either of the line segments joining opposite vertices.

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

A square is a special case of a rectangle where all of the sides are of equal length.

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