Icosahedron

A Maths Starter of The Day

How Many Triangles?

 

 

 

 

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Keywords: Starter | Shape (3D)

  • Mike Litoris, Mianus, Nebraska
  • Wednesday, March 04, 2009
  • "Any DnD (Dungeons & Dragons) player will immediately recognize this familiar shape as a d20. A good task is to ask the students to form the net, or better yet, make there own."
  • Gopal Singh Porte, Kurukshetra
  • Monday, August 30, 2010
  • "I think there are 20 triangle."
  • Best Year Maths Group, King Alfred's College The Best
  • Tuesday, March 01, 2011
  • "Really hard but a good starter."
  • Mr Cansdale, Wintringham Grimsby
  • Thursday, May 19, 2011
  • "The majority of 7Y1 think 25 (Juris got 20 instantly)."
  • Barbara Clark, The Plume School
  • Tuesday, June 07, 2011
  • "I would like to be able to stop it moving so that students can see where the sides and edges are. The diagram is excellent, but quite hard for students to really see, unless they understand immediately."
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  • Transum,
  • Saturday, June 11, 2011
  • "Thanks for your comments Barbara. Use the freeze button on your projector's remote control to stop the diagram moving."

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This starter has scored a mean of 3.7 out of 5 based on 210 votes.

 

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Answer
20
(or 40 if you count the reflection)

As the title suggests, the shape is a regular icosahedron, one of the five Platonic solids. It is a polyhedron composed of 20 triangular faces, with five meeting at each of the 12 vertices connected by 30 edges.

Can you sketch a net for this shape?

Have you seen the Great Dodacahedron?

Other shape counting starters:

How Many Squares 1? | How Many Squares 2?
How Many Triangles 1? | How Many Triangles 2? | How Many Triangles 3?
How Many Rectangles? | Rectangles Investigation | Icosahedron

The animation at the top of this page was adapted from one created by Ramandeep Singh and made available on www.Flashkit.com