Wikipedia:Reference desk/Archives/Mathematics/2014 October 30

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October 30

Chamfer/Conway notaion

There is not "chamfer" or "chamfered cube" in "Symmetries of Things." Where did Conway define "chamfer"? — Preceding unsigned comment added by 中川 宏 (talk • contribs) 11:06, 30 October 2014 (UTC)

I think Conway was just using chamfer in its normal sense as a term for sort of shaving off a corner in architecture, cabinet making, road construction, engineering, etc. "Chamfer" is just an English word for cutting off a corner [1], [2], which can be both a noun and a verb. The term is related to the Bevel. For the geometrical sense of "chamfered cube", it is synonymous with Truncation_(geometry). It's an understandable confusion, because many native English speakers don't know the word "chamfer", and many might conclude that it's part of some mathematical nomenclature. SemanticMantis (talk) 15:28, 30 October 2014 (UTC)

Nets of unusal 3D geomertic figures

Does anyone here have a recomended text on how you draw nets of various 3D figures?

In particular nets for cones, Fustrums and related sections? (I'm having a hard time translating a design of a skirt into flat pattern sections.) 15:44, 30 October 2014 (UTC)

I am having trouble with infinity

I am having trouble with thinking about infinite number of chances. It is getting to me. If there is a probability P of selecting a particular dress and if I reject dress #1, I look at dress #2.

So if there are infinity number of different model of dresses, then the probability of picking/buying one dress is 1.

But if the probability of picking dress #N is P_N and P_1 = x and P_(N) = P_(N-1) / 10

Then according to my calculations, the the probability of picking/buying one dress is less than 1.

So now I have two possible answers... 1 and less than 1 so I am confused. 202.177.218.59 (talk) 23:38, 30 October 2014 (UTC)

Assuming that the probability of selecting a dress at each step is p (e.g., it is determined by a random coin toss), the probability of picking a dress after n steps or less is $1-(1-p)^{n}$, which tends to 1 as n tends to infinity (see binomial distribution. I assume something is wrong with your reasoning beginning on the line "But if the probability of picking dress #N...", although I cannot really make sense of what you mean there. Sławomir Biały (talk) 00:38, 31 October 2014 (UTC)

That's strange, because infinity is working just fine for me. Have you tried turning it off and then on again? Plasmic Physics (talk) 02:46, 31 October 2014 (UTC)

You wrote, "there is a probability P of selecting a particular dress". It's not clear what you mean here. If you mean, "the probability that I will select a particular dress, given that I am looking at it, is a constant P", then yes, the probability that you will select a dress is 1. On the other hand, if the probability of selecting a given dress under consideration can vary with the dress, then the probability of selecting a dress may be less than 1.

Later, you say $P_{n+1}=P_{n}/10$ and $P_{1}=x$. If you mean for a constant P as discussed above, then a little calculation shows that $x=.9$. If you redo your calculations with this, you should get 1.--80.109.80.31 (talk) 11:43, 31 October 2014 (UTC)