A TRAPEZOID is a quadrilateral in whichtwo 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. 17-15.)
Figure 17-15.-Typical trapezoids.
The area of a trapezoid may be foundby separating it into two triangles and a rectangle, as in figure 17-16. The total area A of the trapezoid is the sum of A1 plus A2 plus A3, and is calculated as follows:
Thus the area of a trapezoid is equal to onehalf the altitude times the sum of thebases.
Figure 17-16.-Area of a trapezoid.
Practice problems. Find the area of each ofthe following figures:
1. Rhombus; base 4 in., altitude 3 in.
1. 12 sq in.
The mathematical definition of a circle statesthat it is a plane figure bounded its diameter.
Parts of a Circle
The CIRCUMFERENCE of a circle is theline that forms its outer boundary. Circumference is the special term used in referring to radius at the point of tangency.
Figure 17-17.- Parts of a circle.
An ARC is a portion of the circumference ofa 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. 17-18.)