Composite Shapes Reasoning to Find Area

Student Facing Goals I can use different reasoning strategies to find the area of shapes. Lesson Narrative This lesson is the third of three lessons that use the following principles for reasoning about figures to find area: If two figures can be placed one on top of the other so that they match up exactly, then they have the same area. If a figure is composed from pieces that don't overlap, the sum of the areas of the pieces is the area of the figure. If a given figure is decomposed into pieces, then the area of the given figure is the sum of the areas of the pieces. Following these principles, students can use several strategies to find the area of a figure. They can: Decompose it into shapes whose areas they can calculate. Decompose and rearrange it into shapes whose areas they can calculate. Consider it as a shape with one or more missing pieces, calculate the area of the shape, then subtract the areas of the missing pieces. Enclose it with a figure whose area they can calculate, consider the result as a region with missing pieces, and find its area using the previous strategy. Use of these strategies involves looking for and making use of structure (MP7); explaining them involves constructing logical arguments (MP3). For now, rectangles are the only shapes whose areas students know how to calculate, but the strategies will become more powerful as students’ repertoires grow. This lesson includes one figure for which the “enclosing” strategy is appropriate, however, that strategy is not the main focus of the lesson and is not included in the list of strategies at the end. Note that these materials use the “dot” notation (for example ) to represent multiplication instead of the “cross” notation (for example ). This is because students will be writing many algebraic expressions and equations in this course, sometimes involving the letter  used as a variable. This notation will be new for many students, and they will need explicit guidance in using it.

By Todd Steinhauer