Course Guide - USAID Teacher Education Project

Unit 3 GeometryWeek 3, Session 2: Geometric Measurement, Perimeter of polygons andirregular shapes1. What are the important concepts?a) Perimeter is a one-dimensional (linear) measurement that surrounds a twodimensionalfigure or three-dimensional object.b) Perimeter can be measured around irregular figures, polygons, circles, and ellipses(and three-dimensional objects).c) Various methods and tools can be used to find perimeter.d) Formulas for finding perimeter may be expressed in various ways.e) While it may be relatively easy to estimate the area of certain basic shapes (such astriangles, trapezoids, and parallelograms other than squares and rectangles), it issignificantly more difficult to estimate their perimeter.This is because these shapes have sides that do not meet at right angles. (Or aschildren would say, sides that are "slanted.")f) If students inquire about the perimeter (circumference) of circles, note that thisconcept will be explored in detail next week.2. How do children think about these concepts?a) If perimeter is introduced by measuring and adding the dimensions of squares andrectangles or by using a formula, children tend to assume that perimeter is acharacteristic of polygons.This is why it is important to begin with the generalized concept of perimeter as themeasurement around any shape. Thus, this topic begins with measuring the perimeterof irregular shapes and then moves to discovering the area of various polygons.(Later in this unit students will investigate the perimeter of circles, a measurement wecall circumference.)b) Textbooks often label shapes with their dimensions, making it relatively simple forchildren to add the side lengths to find the perimeter.However, this is not how measuring perimeter occurs in the real world. In everydaysituations involving perimeter, the dimensions are not known. They need to bemeasured by using various tools: rulers or tape measures, string for irregular shapes(which then needs to be measured by a ruler or tape measure), and for large shapes(such as the classroom floor) by a tool such as a trundle wheel.