A quadrilateral is defined as a closed figure with four straight sides. The definition of a closed figure is illustrated with examples of closed and non-closed figures. Examples of closed figures are used to illustrate the definition of a quadrilateral, with examples that are quadrilaterals, and figures that are not quadrilaterals.
The non-examples include figures with curved sides instead of straight sides, figures with more or fewer sides than four, and figures that are not closed. This set of examples is designed to make sure that students understand the features of figures necessary to define the concept of a quadrilateral.
Students then identify figures that are quadrilaterals by selecting quadrilaterals from a set of figures that include quadrilaterals and figures that are not quadrilaterals because of the absence of one or more required features of a quadrilateral.