# 3d shape definitions - when is a face a face and an edge and edge??

Discussion in 'Mathematics' started by Tomzy, Oct 12, 2010.

1. ### TomzyNew commenter

When teaching 3d shapes at KS2 I have always taught that a face is a surface of a shape, e.g. a cube has 6 square faces, a cylinder has 3 faces, 2 circular and, if you 'unwrapped' it, 1 rectangular. It would therefore follow that a cube has 8 edges and a cylinder has 2, as an edge is where 2 faces meet. However, on looking at maths definitions on the internet, I read that a face is only the FLAT surface of a 3d shape and a face is NOT a curved surface. Therefore a cylinder has only 2 faces and as the faces do not meet, no edges.
I've checked the QCA glossary and they also seem to say the same - defining an edge as where 2 plane surfaces meet.
However (again) the other Y6 teacher disagrees with the above and agrees with my previous explanantion ( hope you're all following this)! As a result, we have both taught different definitions, throwing us, and the poor children, into complete confusion!

2. ### ResourceFinder

The curved part is part of the surface but I agree ... not a face

Then again, what do I know ... I still want it to be a prism

3. ### lilachardyStar commenter

A cube has 12 edges.

4. ### ResourceFinder

oooops I missed that

5. ### TomzyNew commenter

so did I! But it doesn't answer my question!

6. ### Jeanie-JuNew commenter

I would say it has 3 faces - which would include the curved. I think as far as (silly)SATs & the like is concerned that would be what I would say to the children and they could then delve more deeply when older. I am, however, a mere mortal.
Anyone know the decision behind heptagon and septagon??? I have been marking my children's work where they have labelled a seven sided shape as a septagon and I have never heard of it before (I promise I'm not completely stupid)!!! I have looked it up and it is a used word with the same definition but I wondered if there is a general idea of if it is an American-used word /old terminology /disliked term????????
I have read of the origins but that doesn't help me to understand the use 'today'. Should i teach both?? Drop one??? I don't like 'septagon'. ;-(

7. ### ResourceFinder

No, but I answered it in post 2

8. ### ResourceFinder

septagon fits so nicely with octagon, nonagon, and decagon

9. ### DMNew commenter

You can mark it correct. Either word is fine.
I'm sure linguists would complain that septagon is a bit mangled as it has a Latin prefix and a Greek suffix.

10. ### NazardNew commenter

I must confess that it always feels slightly wrong to say "nonagon" for the same reason.
But mathsisfun is happy with "septagon", so that settles it!

11. ### DMNew commenter

mathsisfun has spoken.

12. ### kevchenko13

http://www.mathsisfun.com/definitions/hendecagon.html
What about the hendecagon versus undecagon issue? 'Hen-' has the Greek advantage over the Latin 'un-' here, so fits with 'hepta-', but confusingly also has the backing of mathsisfun where 'hepta-' didn't. Now I don't know what to think!

13. ### Polecat

I call a 7-sided figure a 7-sided figure.
No one studies Latin or Ancient Greek now,
so lets make it easy for the kids.

14. ### DMNew commenter

Maybe this is true of Ancient Greek but certainly not Latin. We now offer GCSE Latin (as an afterschool enrichment activity) in a bog standard comp - we had no choice really because all our feeder schools have Latin clubs.
100,000 copies of Minimus have been sold to primary school children.
http://www.minimus-etc.co.uk/

15. ### NazardNew commenter

You don't need to think - just read mathsisfun and believe!

16. ### DMNew commenter

Teachers and students come and go but the word of mathsisfun is eternal.

17. ### NazardNew commenter

I had a moment of doubt and looked for an alternative to the Scripture According to Mathsisfun.
I found this funny little website called "Wolfman MathWorld" (or something). It's really good that Were-wolves are able to be given these jobs and are able to be part of society, but I had to laugh when I saw the following:
Heptagon: It is also sometimes called a septagon, though this usage mixes a Latin prefix sept- (derived from septua-, meaning "seven") with the Greek suffix -gon (from gonia, meaning "angle"), and is therefore not recommended.
A hendecagon is an 11-sided polygon, also variously known as the undecagon or unidecagon. The term "hendecagon" is preferable to the other two since it uses the Greek prefix and suffix instead of mixing a Roman prefix and Greek suffix.
Right - so mixing Latin and Greek is <u>bad</u> (things like "television" - won't ever catch on...).
Imagine my surprise, etc, when I read this entry:
The nonagon, also known as an enneagon, is a 9-sided polygon. Although the term "enneagon" is perhaps preferable (since it uses the Greek prefix and suffix instead of the mixed Roman/Greek nonagon), the term "nonagon," which is simpler to spell and pronounce, is used in this work.
Consistency, conshmistency!
I have learned my lesson, have purged myself with hyssop and am back in the mathsisfun fold for good. Hail mathsisfun! Hail mathsisfun!