Lesson 9

In previous grades, students learned formulas for the volumes of cones, cylinders, and spheres. In this lesson, they recall how to calculate the volume of a cylinder, using informal arguments to compare the volume of a cylinder to the volume of a prism that has an equal height and area of its base. Students start to think about volume in 1-unit layers. This lays a foundation for the informal derivation of the pyramid formula, which will rely on consideration of area of cross sections. Finally, students apply cylinder volume calculations to a solid of rotation.

Students have opportunities to reason abstractly and quantitatively (MP2) when they compare prism and cylinder volumes.

If necessary here and in the remainder of the unit, provide students with formulas for the areas of triangles, parallelograms, and circles. Students developed and worked with these formulas in earlier grades. Note that we use “area of a circle” as shorthand for “the area of the region enclosed by a circle.”