Coins and Maths

I like coins. I’m not a collector. The only coins I have in my possession are the odd foreign euro or dirham from holidays gone by, and a big pile of various UK coins. I’ve never sought a coin in particular, but maybe after this post I will.

I’m focusing, unsurprisingly, on the maths of coins. They come in all sorts of amazing mathematical shapes. I thought I’d show some of them to you. Let’s see how many polygons we can get through. I won’t bother with circles, because duh.

3: The Triangle

There is an Australian $5 coin! This is not generally circulated currency, but rather a commemorative mint to mark the 25th anniversary of Australian Parliament House. Still, I want one.
2013-$5-Silver-Proof-Parliament-House-Triangular_OBV-Thumb
Another commemorative mint was this Isle of Man coin celebrating the archeologist Howard Carter
2009-Tutankhamun-Sand-Triangle-Coin
Then there’s this great Reuleaux Triangle coin from Bermuda, which I *think* was general circulated currency.
1ber1YRcn
You may have noticed that the reuleaux triangle is one of the family of shapes of constant width, known collectively as the reuleaux polygons. The twenty pence piece is one too. Many coins are, as they roll nicely in slot machines.
4: The quadrilateral

Another commemorative Australian Coin (a squircle!)

2002KookaburraSquare_5631

The Netherlands (generally circulated):

netherlands_antilles_5_cents_1965

Poland (commemorative)

square-coin

coin7_0

I think there was an old Indian Rupee that was square too.

Or how about this rectangular Galapagos $8 coin? No idea whether this was in circulation or commemorative. You’d think for a teeny place like that… well who knows.

galapagos8r

There’s also a rectangular commemorative Korean coin:

L3105

5: The pentagon

Slovakia (circulated):

10000sk

Fiji (commemorative):

6975792846_c0e49c2c9d

6: Hexagonal

Egypt (1940s)

egypt_2_piastres_1944

Caribbean (commemorative)

carib

7: Heptagonal

50p (UK)

decimalisation50pdesigns

Falkland Islands:

Falk20p

Barbados:

Barbados1-1973

Aah there’s loads of heptagonal coins. Let’s move swiftly on.

8: Octagonal

Maltese 25 cent coin:

547d3698e1198_221168b

9: Nonagonal

Kenyan 5 shillings:

a6bca536-10f8-11e3-9006-7287628a0eb7

Austria (commemorative)

Austrian-Commemorative-Joseph-Haydn-Nine-Sided-5-Euro-Coin

10: Decagonal

Belize:

images

Jamaica:

jama65

11: Hendecagonal

We’re learning new words now!

Sri Lanka:

20111

12: Dodecagonal

UK Pound Coin (Coming 2017!)

one_pound_heads

UK 3 pence (back in the day!)

thrupenny_bit

13: Tridecagonal

Czech Republic:

czech_republic-20-korun-1993

14: Tetradecagon

We’re getting silly now!

Australian (commemorative):

5cb3956228a2f69b06037107c12477b7

Phew! We’re practically full circle (pun intended).

I can’t find a coin with more sides than this. Can you?

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