Surface area of rectangular prisms 2

Surface area. Sample problem here.

Text goes here Movie goes here

 To measure the area of a 2-dimensional object, you need to count the number of unit squares necessary for covering the object. A unit square can be 1" x 1" or 1 yd x 1 yd or 1 ft x 1 ft or a square by some other unit.

Unit Square

To find the area of a 3 cm-by-5 cm rectangle, you would imagine that rectangle cut into unit squares. Then you would count the squares.

Of course, a short cut for finding the area of a rectangle is to multiply the length by the width.

A = length • width

A = lw

For the surface area of a solid, there is a similar definition, but it applies to the exterior surfaces of the solid. Surface area is the sum of all unit squares that fit on the exterior of a solid.

 

As the diagram below indicates, there are six surfaces to a rectangular prism. There is a front, back, top, bottom, left, and right to every rectangular prism. The surface are of a prism is nothing more than the sum of all the areas of these rectangles.

Notice that the top and bottom faces will have equal areas. The front and back faces will have equal areas. Finally, the left and right faces will have equal areas. This means instead of finding the area of all six faces, we only need to find the areas of three faces, multiply each by 2, and then find their sum.

Surface Area = Top + Bottom + Front + Back + Left + Right

Surface Area = 2 x Top + 2 x Front + 2 x Left

Surface Area = 2lw + 2lh + 2wh

 

Surface area of rectangular prism

Surface Area = $2lw + 2lh + 2wh$

 

 

Example 1

Find the surface area for this rectangular prism.

SA = 2 • Top + 2 • Front + 2 • Left

SA = 2lw + 2lh + 2wh

SA = 2 • 4 • 3 + 2 • 4 • 5 + 2 • 3 • 5

SA = 24 + 40 + 30

SA = 94 cm^2

 

 

 

Example 2

Find the surface area for this rectangular prism.

SA = 2 • Top + 2 • Front + 2 • Left

SA = 2lw + 2lh + 2wh

SA = 2 • 6 • 3 + 2 • 6 • 3 + 2 • 3 • 3

SA = 36 + 36 + 18

SA = 90 cm^2

 

 

 

Example 3

Find the surface area for this rectangular prism.

SA = 2 • Top + 2 • Front + 2 • Left

SA = 2lw + 2lh + 2wh

SA = 2 • 5 • 4 + 2 • 5 • 7 + 2 • 4 • 7

SA = 40 + 70 + 56

SA = 166 cm^2

 

Geogebra applet:

Use the length, width, and height sliders to create a rectangular prism of your choosing. Notice how the net on the right changes as the move the sliders.

 

Sorry, the GeoGebra Applet could not be started. Please make sure that Java 1.4.2 (or later) is installed and active in your browser (Click here to install Java now)

 

Duane Habecker, Created with GeoGebra

 


 

Self-Check


Question 1

Find the surface area for this rectangular prism.

 

[show answer]


Question 2

Find the surface area for this rectangular prism.

 

[show answer]


Question 3

Find the surface area for this rectangular prism.

 

[show answer]


 

 

 

 

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