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Follow instructions to fold sheets of A4 paper into pentagons and assemble them to form a dodecahedron. Calculate the error in the angle of the not perfectly regular pentagons you make.

Two Regular Polygons

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

Simon from Elizabeth College, Guernsey found solutions using a spreadsheet :

We have made up a spreadsheet from Simon's description, the columns are :

  • number of sides for Polygon A

  • exterior angle for Polygon A is 360/n

  • exterior angle for Polygon B is 81 - column B

  • number of sides required for that exterior angle is 360/column C

Here's the spreadsheet file : Two Regular Polygons

Simon continues :

Polygon A starts with few sides and increases, Polygon B is calculated for each case.

As A increases in side number, B must decrease its own side number.

So once A exceeds B no new solutions will be found.

For a target angle of 81 I found two solutions : 5-40 and 8-10.

I then tried it with a target angle of 54

This gave me four solutions : 7-140, 8-40, 10-20 and 12-15

Finally I tried with a target angle of 27

This gave me five solutions : 14-280, 15-120, 16-80, 20-40 and 24-30

Simon then summarised the situation:

As the number of sides doubles, the angle is halved, and in general : the number of sides is in inverse proportion to the exterior angle.

27 is one third of 81 so it includes solutions that are three times the solutions for 81.

54 is double 27 so it has solutions which are half the size of solutions for 27 provided that value is an integer.

Excellent thinking and skilful application of a spreadsheet Simon, very well done.