Quote:

- How many
coin flipsare expected for the first tail?- is number of
coin flipsbefore first tail. What is 's dispersion, ?

- ... but what is ? The answer given is .

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- Sep 15th 2010, 09:05 AMcourteousHow many coin flips until first tail (or head)Quote:

- How many
__coin flips__are expected for the first tail? - is number of
__coin flips__before first tail. What is 's dispersion, ?

- ... but what is ? The answer given is .

- How many
- Sep 17th 2010, 11:32 PMmatheagle
I'm guessing that this is a geo with p=.5.

X is the trial in which the first heads appears?

D would be the variance then, also known as the dispersion. - Sep 18th 2010, 07:02 AMcourteous
Yes, but how do you get (which is the answer) for the second point/question?

- Sep 18th 2010, 07:06 AMmatheagle
The expectation of a geo is 1/p.

That's easy to prove and I'm sure it's online too.

It's obvious too, since the smaller p is, the longer one should wait until the the success arrives.

Geometric distribution - Wikipedia, the free encyclopedia

http://cnx.org/content/m13124/latest/ - Sep 18th 2010, 07:51 AMcourteousQuote:

- How many
__coin flips__are expected for the first tail? - is number of
__coin flips__**before**first tail. What is ?

- Solved this one. (Nod)
- Don't know how to find dispersion.

- How many