For every polyhedron there is a construction for a so called dual polyhedron by connecting together the centres of the faces. The diagram on the left below shows this construction for a cube that constructs an octahedron inside it. 2²³ B .O Choose an origin O to be the centre of the base of the cube and choose the i, j and k directions to be parallel to the faces of the cube. Let A be the centre of the face with positive i-coordinate and B be the centre of the face with positive j- coordinate, as shown in the right-hand diagram. Let the cube have a side length 2. (a) Write down OA and OB in terms of the unit vectors i, j and k. (b) Calculate OÀ OB and hence calculate the angle AOB. (c) Calculate OÀ X OB and hence calculate the ratio of the surface area of the octahedron to the surface area of the cube.

Elementary Geometry For College Students, 7e
7th Edition
ISBN:9781337614085
Author:Alexander, Daniel C.; Koeberlein, Geralyn M.
Publisher:Alexander, Daniel C.; Koeberlein, Geralyn M.
Chapter9: Surfaces And Solids
Section9.2: Pyramids, Area, And Volume
Problem 6E
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For every polyhedron there is a construction for a so called dual
polyhedron by connecting together the centres of the faces. The
diagram on the left below shows this construction for a cube that
constructs an octahedron inside it.
j
"K³
B
i
0
Choose an origin O to be the centre of the base of the cube and choose
the i, j and k directions to be parallel to the faces of the cube. Let A be
the centre of the face with positive i-coordinate and B be the centre of
the face with positive j- coordinate, as shown in the right-hand diagram.
Let the cube have a side length 2.
(a) Write down OA and OB in terms of the unit vectors i, j and k.
(b) Calculate ŌÀ OB and hence calculate the angle AOB.
(c) Calculate OÀ X OB and hence calculate the ratio of the surface
area of the octahedron to the surface area of the cube.
Transcribed Image Text:For every polyhedron there is a construction for a so called dual polyhedron by connecting together the centres of the faces. The diagram on the left below shows this construction for a cube that constructs an octahedron inside it. j "K³ B i 0 Choose an origin O to be the centre of the base of the cube and choose the i, j and k directions to be parallel to the faces of the cube. Let A be the centre of the face with positive i-coordinate and B be the centre of the face with positive j- coordinate, as shown in the right-hand diagram. Let the cube have a side length 2. (a) Write down OA and OB in terms of the unit vectors i, j and k. (b) Calculate ŌÀ OB and hence calculate the angle AOB. (c) Calculate OÀ X OB and hence calculate the ratio of the surface area of the octahedron to the surface area of the cube.
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