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Trapezoids A TRAPEZOID is a quadrilateral in which two sides are parallel and the other two sides are not parallel. By orienting a trapezoid so that its parallel sides are horizontal, we may call the parallel sides bases. Observe that the bases (See fig. 1715.) Figure 1715.Typical trapezoids. The area of a trapezoid may be found by separating it into two triangles and a rectangle, as in figure 1716. The total area A of the trapezoid is the sum of A_{1} plus A_{2} plus A_{3}, and is calculated as follows: Thus the area of a trapezoid is equal to onehalf the altitude times the sum of the bases. Figure 1716.Area of a trapezoid. Practice problems. Find the area of each of the following figures: 1. Rhombus; base 4 in., altitude 3 in. Answers: 1. 12 sq in. CIRCLES The mathematical definition of a circle states that it is a plane figure bounded its diameter. Parts of a Circle The CIRCUMFERENCE of a circle is the line that forms its outer boundary. Circumference is the special term used in referring to radius at the point of tangency. Figure 1717. Parts of a circle. An ARC is a portion of the circumference of a circle. A CHORD is a straight line joining the end points of any arc. The portion of the area of a circle cut off by a chord is a SEGMENT of the circle, and the portion of the circle’s area cut off by two radii (radius lines) is a SECTOR. (See fig. 1718.) 