Finding 3D stacks

Can you find a way of counting the spheres in these arrangements?

Problem

This activity may be used as a follow-on for those who have successfully worked at Doplication.

 

Let me help you visualise this representation of a 3D situation.

 

The picture below shows a 2 by 3 by 4 arrangement of red spheres, which you might see as two layers of 3 by 4:

 

Image
Image described above.

 

These two layers together could also be viewed as representing six cubes:

 

Image
Six cubes with red spheres on their vertices, stuck together in a 3-by-2-by-1 arrangement.

 

 

In the top picture the centre of each cube is shown with a light or dark brown sphere. 
 

How many spheres are there altogether in this 2 by 3 by 4 arrangement?

 

How did you count them?

 

Use your system of counting to find the total number of spheres for other sizes, for example 3 by 3 by 3; 3 by 4 by 4; 4 by 4 by 5 etc.

 

How would you find the number of spheres for any sized arrangement?