# 7.05 Cross sections of prisms and pyramids

Lesson
A cross-section is the 2D shape we get when cutting straight through a 3D object. For example, the orange below (let's pretend it's a perfect sphere) has been cut along the vertical plane (straight up and down). The cross-section of the orange, where we can see inside, is a circle.

When we slice a piece of fruit, we see its cross-section.

#### Exploration

Let's use the applet below to explore cross-sections that are parallel to the base of a prism and pyramid with the same polygon base.

1. Drag the blue point in the 3D view up and down. What do you notice about the cross-section of the pyramid compared to the cross-section of the prism?
2. Move the slider at the bottom to change the number of sides. Do your observations change when you increase the number of sides for the base?

Now let's explore some cross-sections that are perpendicular to the base of a prism and pyramid with the same polygon base.

1. Drag the blue point in the 3D view to move the plane from left to right. What do you notice about the cross-section of the pyramid?
2. What do you notice about the cross-section of the prism?
3. Move the slider at the bottom to change the number of sides. Do your observations change when you increase the number of sides for the base?
4. How do the vertical cross-sections compare to the horizontal cross-sections of the same type of solid?

Recall that prisms have rectangular sides, and the shape on the top and the base is the same. The name of the base shape gives the prism its name.

Prisms
Triangular Square Rectangular Pentagonal Hexagonal Octagonal

Any cross-section taken parallel to the base in a prism is always the same shape and size as the base. In other words, we say that a prism has a uniform cross-section.

Recall that pyramids have triangular sides, and the shape of the base gives the prism its name.

Pyramids
Triangular Square Rectangular Pentagonal Hexagonal Octagonal

In a pyramid, any cross-section taken parallel to the base is always the same shape but is smaller in size than the base.

From the applets, we can see that three-dimensional shapes have more than one cross-section and they may or not be the same shape. It all depends on which way we cut them!

#### Practice questions

##### Question 1

We want to classify the following solid:

1. Does the shape have a uniform cross-section ?

Yes

A

No

B
2. The solid is a:

Triangular Pyramid

A

Rectangular Prism

B

Rectangular Pyramid

C

Triangular Prism

D

##### Question 2

Consider the solid in the adjacent figure.

1. If the solid is cut straight down below the dotted line, what cross-section results?

A pentagon

A

A triangle

B

A hexagon

C

A square

D
2. Does the solid above have a uniform cross-section?

Yes

A

No

B
3. What is the name of the solid?

A triangular prism

A

A rectangular prism

B

A hexagonal prism

C

A pentagonal prism.

D

##### Question 3

Which two of the objects below could have the following cross-section?

1. Triangular Pyramid

A

Cylinder

B

Pentagonal Pyramid

C

Pentagonal Prism

D

### Outcomes

#### 7.G.3

Describe the two-dimensional figures that result from slicing three-dimensional figures, as in plane sections of right rectangular prisms and right rectangular pyramids.