### At a Glance

The area of a regular pentagon looks about twice as a big as the pentangle star drawn within it. Is it?

### Pent

The diagram shows a regular pentagon with sides of unit length. Find all the angles in the diagram. Prove that the quadrilateral shown in red is a rhombus.

### Pentakite

ABCDE is a regular pentagon of side length one unit. BC produced meets ED produced at F. Show that triangle CDF is congruent to triangle EDB. Find the length of BE.

# Dodecawhat

##### Stage: 4 Challenge Level:

We are going to make a pentagon. By making twelve such pentagons you can construct a dodecahedron like the one in the picture.

For each pentagon, you will need a piece of A4 paper and then follow the instructions below.

Fold the paper in half both ways to find the centre O. Fold along the red line so A touches O. Fold C to O similarly. Fold B and D to O.   Next fold along PQ. As the two halves come together,tuck the flap from corner D behind the flap from corner B to make 'pockets' (see the diagrams below).

Fold R and S up to the centre line EO, so that they meet to form a straight line and make a pentagon.

If you make 12 pentagons in this way and assemble them, using your 'flaps' and 'pockets', you can make a dodecahedron.

#### Now for the problem:

If you use A4 paper for this construction and try to make regular pentagons there is a small error in the angle at E. Find this error and find the dimensions of the paper which you would need to use to get an accurate regular pentagon and hence an accurate regular dodecahedron.

A4 paper has sides in the ratio$\sqrt2$ to $1$.Prove that when you fold A4 paper in half (to get A5) or in quarters (to get A6) the rectangles you get have side lengths in the same ratio.

Extra Resources:

1) You can construct other platonic solids using paper and this article explains how.

2) Have a look at the October 2000 Article titled Classifying Solids using Angle Deficiency

3) You can download a demo version of Stella , a computer program which lets you create and view polyhedra on the screen, then print out the nets required to build your own models out of paper. Small Stella and Great Stella are available from the Stella Website.

4) Alternatively, print out the models from this this pdf available at the British Crystallographic Association's Website

The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice. More information on many of our other activities can be found here.
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