# Volume Formulas, Cavalieri's Principle, and 2-D Cross-Sections

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## Objective

Students will be able to identify the shapes of 2D cross-sections of 3D objects and use geometric shapes, their measures, and properties, to describe objects.

#### Big Idea

In a differentiated lesson based on student choice, students will be able to practice finding the volume of prisms or apply Cavalieri's Principle to determine the volume formula for a sphere.

## Homework Review

20 minutes

To begin class today I ask my students to compare their work on the Origami Box Homework with others, making corrections and changes in a colorful pen.  Since this homework requires students to exercise algebraic manipulation, it is really important for them to have a chance to explain how they performed their procedures and made sense of algebraic expressions to each other (MP3).

When I debrief this homework assignment with the whole class, I make sure to call attention to one or two of the Origami Box posters to help students recall how similar solids’ volumes and surface areas increase by the scale factor3 or scale factor2.

## Launch Volume

25 minutes

I use this powerpoint to formally launch this part of my Measurement and Dimensionality unit.  Since my students have had previous experiences dealing with volume, I want to activate their prior knowledge and create intrigue by introducing them to Cavalieri's Principle right from the beginning, in a motivating candy bar context.

## You Choose: Practice Thinking About Volume or Take it Further

25 minutes

Since students have seen volume before in their elementary and middle school experiences, they exhibit a range of comfort levels with solving volume problems.  For this reason, it is important to differentiate students' work to engage students at appropriate levels.  In this lesson, I give students the choice between practicing calculating the volume of prisms (triangular, rectangular, pentagonal, and hexagonal), and "taking it further," where we will apply Cavalieri's Principle to determine the volume formula for a sphere (MP8).

Since I facilitate the "taking it further" small group discussion while other students work on “Thinking About Volume,” I make sure to physically position myself so I can easily monitor students as they work.  After I ensure students’ understand the comparison of the cross-sections of the hemisphere with the cross-sections of the cylinder containing the cone, I leave the group to work out the algebra while I check in with the students who are calculating the volumes of the prisms they have built.

## Exit Ticket: 2-D Cross Sections

5 minutes

I want to assess students’ understanding of Cavalieri’s Principle and their facility with determining the 3-D solids made by 2-D cross-sections.  I ask students to work on this check for understanding individually to give me an accurate picture of students’ understanding.  I collect these exit tickets as students leave the classroom, selecting model student writing samples to share out with the class in the next lesson.

## Homework: Cavalieri's Principle and 2-D Cross-Sections

5 minutes

This homework assignment gives students the opportunity to consider cross-sections of 3D figures and to solve volume problems.