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Sunday, May 4, 2014

More Cube Problems, 6

Category: Solid Geometry, Plane Geometry

"Published in Vacaville, California, USA"

Pass a plane through a cube so that the section formed will be a regular hexagon. If the edge of the cube is 2 units, find the area of this section.

Solution:

To illustrate the problem, it is better to draw the figure as follows

Photo by Math Principles in Everyday Life

The intersection of a cube with a cutting plane is a regular hexagon with 2 units of its sides. The vertices of a regular hexagon are located at the midpoint of six sides of a cube. By Pythagorean Theorem, we can calculate the sides of a regular hexagon as follows


 




Next, analyze the section as follows

Photo by Math Principles in Everyday Life

Since this is a regular hexagon, then we can calculate the vertex angle of the six triangles as follows



The six triangles are all isosceles triangles because the above figure is a regular hexagon. Let's calculate the base angles of an isosceles triangle as follows





Since all angles of an isosceles triangle are all equal, then all six triangles of a regular hexagon are equiangular or equilateral triangles. 

Photo by Math Principles in Everyday Life
  
By using Pythagorean Theorem, the altitude of an equilateral triangle is
 
 







The area of a triangle is







Therefore, the area of a regular hexagon which is the section of a cube is



Saturday, May 3, 2014

More Cube Problems, 5

Category: Solid Geometry

"Published in Vacaville, California, USA"

One cube has a face equivalent to the total area of another cube. Find the ratio of their volumes.

Solution:

To illustrate the problem, it is better to draw the figure as follows

Photo by Math Principles in Everyday Life

Let B = be the area of a base of a large cube
      x = be the length of the edge of a large cube
      T = be the total area of a small cube
      y = be the length of the edge of a small cube
      V1 = be the volume of a large cube
      V2 = be the volume of a small cube 

The area of a base of a large cube is


The total area of a small cube is

 
From the given problem statement, we know that
 
 



The volume of a large cube is


 
 
 

The volume of a small cube is


Therefore, the ratio of their volumes is


 

Friday, May 2, 2014

More Cube Problems, 4

Category: Solid Geometry, Plane Geometry

"Published in Newark, California, USA"

Find the area of a triangle whose vertex is at the midpoint of an upper edge of a cube of edge a and whose base coincides with the diagonally opposite edge of the cube.

Solution:

To illustrate the problem, it is better to draw the figure as follows

Photo by Math Principles in Everyday Life

The first thing that we need to do is to solve for the altitude of a triangle first. Since one of the vertex of a triangle is located at the midpoint of the upper edge of a cube, then a triangle is an isosceles triangle. The altitude of an isosceles triangle is equal to the hypotenuse of an isosceles right triangle at the right side of a cube. 

By Pythagorean Theorem






Therefore, the area of a triangle is