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# Prisms and Cylinders

Sep 13, 2022

## Key Concepts

• Use the net of a prism.
• Find the surface area of a right prism.
• Find the surface area of a cylinder.
• Find the height of a cylinder.

## Introduction

### Prism

Prism is a polyhedron with two congruent faces, called bases, that lie in parallel planes.

#### Lateral faces

All faces other than the bases; are parallelograms formed by connecting the corresponding vertices of the bases.

#### Lateral edges

Segments connecting the vertices of the bases and lateral faces.

#### Surface area: (of a polyhedron)

Sum of the areas of its faces.

#### Lateral area: (of a polyhedron)

Sum of the area of its lateral faces.

Net: Two-dimensional representation of the faces.

Right prism: Each lateral edge is perpendicular to both bases.

Oblique prism: Lateral edges are not perpendicular to the bases.

### Use the net of a prism

Example 1:

Find the surface area of a rectangular prism with a height of 3 inches, length 7 inches, and width 4 inches using a net.

Solution:

Step 1: Sketch the prism. Imagine unfolding it to make a net.

STEP 2: Find the areas of the rectangles that form the faces of the prism.

STEP 3: Add the areas of all the faces to find the surface area.

The surface area of the prism is S = 2(21) + 2(12) + 2(28) = 122 inch2

### Find the surface area of a right prism

The surface area of a prism is

• S = Ph + 2B, where
• P = perimeter of the base
• h = height
• B = area of the base

Example 2:

Find the surface area of a right rectangular prism with a height of 3 inches, length 7 inches, and width 4 inches using the formula for the surface area of a right prism.

Solution:

Surface area = 2B + Ph

B = 7 × 4 = 28

P = 2L + 2W

= 2(7) + 2(4)

= 22

h = 3

S.A = 2(28) + 22(3)

= 122 inch2

### Find the surface area of a cylinder

Cylinder: Solid with congruent circular bases that lie in parallel planes. The height is the perpendicular distance between its bases. The radius of a base is the radius of the cylinder.

Right cylinder: The segment joining the centers of the bases is perpendicular to the bases.

Lateral Surface Area: The lateral area of a cylinder is the area of its curved surface. It is equal to the product of the circumference and the height, or 2πrh.

Surface Area: The surface area of a cylinder is equal to the sum of the lateral area and the areas of the two bases.

Example 3:

Find the surface area of the right cylinder.

Solution:

S = 2πr2 + 2πrh (Formula for surface area of a cylinder)

= 2π(32) + 2π(3)(4) (Substitute known values)

= 18π + 24π  (Simplify)

= 42π

= 131.88 (Use a calculator)

The surface area is about 131.88 square feet.

### Find the height of a cylinder

Example 4:

Find the height of a cylinder that has a radius of 6.5 centimeters and a surface area of 592.19 square centimeters.

Solution:

Substitute known values in the formula for the surface area of a right cylinder and solve for the height h

S = 2πr2 + 2πrh (Surface area of a cylinder)

592.19 = 2π(6.5)2 + 2π(6.5)h (Substitute known values)

592.19 = 265.33+ 13πh (Simplify)

592.19 – 265.33 = 13πh (Subtract 265.33 from each side)

326.86 = 40.82h  (Simplify. Use a calculator)

8 ≈ h (Divide each side by 40.82)

The height of the cylinder is about 8 centimeters.

## Exercise

• The surface area of a polyhedron is the sum of the areas of its _____________.
• The lateral area of a polyhedron is the sum of the areas of its ______________________.
• Find the height of the right cylinder shown, which has a surface area of 157.08 square meters.
• Find the surface area of a box of cereal with a height of 15 inches, a length of 8 inches, and a width of 4 inches.
• Find the surface area of a right rectangular prism with a height of 8 inches, a length of 3 inches, and a width of 5 inches.
• Find the lateral area and surface area of the cylinder below.
• Find the surface area of the figure below.
• Find the lateral and surface area of the figure below.
• Draw a net of a triangular prism.
• You are wrapping a stack of 20 compact discs using a shrink wrap. Each disc is cylindrical with a height of 1.2 millimeters and a radius of 60 millimeters. What is the minimum amount of shrink wrap needed to cover the stack of 20 discs?

### What have we learned

• Use the net of a prism to find the surface area.
• Find the surface area of a right prism by using the formula.
• Find the surface area of a cylinder by using the formula.
• Find the height of a cylinder by using the formula.

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