## Reflection: Checks for Understanding Find the Volume of Prisms Using a Formula - Section 3: Guided Practice

As I was watching students work, I kept seeing strange answers for problems GP2 and GP4.  Both of these show prisms that are not "sitting" on their bases.  For GP2, students said the volume of the triangular prism was 60 cubic centimeters.  For GP3, students were saying the volume was 540 cubic meters.

When I asked students how they solved the problem the said "base times tall fills them all".  That of course is the quote of the mnemonic for finding the volume of prisms.  Only it is misapplied.  I had to ask students to tell me the name of the shapes on GP2 and GP4.

I asked them to tell me what the base of the shapes were.  These are triangular prisms but they are "sitting" on rectangles.  It was a bit perplexing;  we have done plenty of work talking about the parts of prisms and how to identify the base.

I asked them how to identify the base of a prism.  Eventually they realized that the bases are the faces that are opposite and parallel to each other.

How Do You Define the Base, Again?
Checks for Understanding: How Do You Define the Base, Again?

# Find the Volume of Prisms Using a Formula

Unit 6: Geometry
Lesson 32 of 37

## Big Idea: Move from the concrete to the abstract. Students turn the modeling of a prism's volume into an algebraic formula.

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45 minutes

### Grant Harris

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