May 3 handout

Regular Polyhedra and the Platonic Solids
 The best known of all the polyhedra are the regular polyhedra, or Platonic solids.
 A regular polyhedron is a convex polyhedron whose faces are congruent regular
polygons, the same number of which meet at each vertex.
 The ancient Greeks proved that there are only five regular polyhedra.
o The tetrahedron has equilateral triangles for faces
o The cube has square faces
o The octahedron has equilateral triangular faces
o The dodecahedron has pentagons for faces
o The icosahedron has equilateral triangular faces
Platonic Solids in Nature
Semiregular Polyhedra
 Some polyhedra have two or more different types of regular polygons for faces.
 Polyhedra whose faces are two or more regular polygons with the same arrangement of
polygons around each vertex are called semiregular polyhedra.
Task 0—Practice while you work
Refer to your “Practicing with Practices” pages and write notes whenever you use a practice
while working today. We’ll share after we do our math tasks.
Task 1-Platonic Solids: Use the Polydrons and build a set of Platonic Solids
 Paper nets for the Platonic Solids are linked to your course Moodle page
Task 2—Relationship between Vertices, Edges and Faces: In the table, record the numbers of
vertices, faces, and edges for each of the Platonic solids you constructed.
Vertices (V)
Edges (E)
Faces (F)
There is a relationship among the numbers of vertices, faces, and edges. Make and record a
conjecture about this relationship and express it in terms of the letters V, F, and E.
Task 3—Truncation Station: If you snip (truncate) the corners (vertices) of the Platonic Solids,
you make a new set of semiregular polyhedra.
a. Use the Polydrons and build each of the following semiregular polyhedra (if you can, or
note that you can’t). Note that the scale changes when you use the Polydrons.
b. Identify the truncated shape (truncated cube, truncated tetrahedron, etc.). Note that you
can snip the vertex “a little way down” or halfway done and get different results.
c. Test your conjecture from Task 2. Does it work?
Task 4: Surface Area of a Sphere—We will try to determine the formula for the surface area of
a sphere using a symmetric piece of citrus fruit.
a. Determine the radius of your piece of fruit. This will be an estimate, since your fruit is
not perfectly spherical.
b. Set the compass opening to the radius of your fruit and draw five circles with the same
radius as your fruit on a blank piece of paper.
c. Peel your piece of fruit, breaking the peel into small pieces. See how many of the circles
can be filled with the pieces. Be sure to leave as little space between the pieces as
possible and to use very flat pieces (break them smaller if they are not flat).
d. You can now see how many circular areas can be covered by the surface (skin) of our
piece of spherical fruit.
The surface area of a sphere of radius r is _________________________.
Task 5: Volume—Cone, Cylinder, Square Prism, Cube and Sphere
a. Measure the polyhedra using the centimeter measuring tapes. Measure the "parts" of the
polyhedral models, as accurately as you can (in cm), that you will need to compute the
volume of each polyhedron.
Needed measurements (in centimeters)
Square Pyramid
b. Compare the polyhedra using your measurements and visual inspection. What
relationships for volume do you think there are?
c. Measure the polyhedral volumes with water, what relationships for volume can you find?