- #1
Nikitin
- 735
- 27
Hi. Assume there's a probability ##q## for a guy to take a step to the right, and ##p=1-q## to take one to the left. Then the probability to take ##n## steps to the right out of ##N## trials is ##P(n) = {{N}\choose{n} }q^n p^{N-n}##.
Now, what is ##<n>##? My textbook in statistical physics says ##<n> = \sum_{n=0}^N n{{N}\choose{n}} q^n p^{N-n} = q \frac{\partial}{\partial q} \sum_{n=0}^N {{N}\choose{n} }q^n p^{N-n} = q \frac{\partial}{\partial q} (q+p)^N = qN##
OK, fair enough.. But what about the fact that ##(q+p)^N=1## ? Shouldn't this mean you're taking the partial derivative of a constant, and thus it equals 0? What am I missing here?
Now, what is ##<n>##? My textbook in statistical physics says ##<n> = \sum_{n=0}^N n{{N}\choose{n}} q^n p^{N-n} = q \frac{\partial}{\partial q} \sum_{n=0}^N {{N}\choose{n} }q^n p^{N-n} = q \frac{\partial}{\partial q} (q+p)^N = qN##
OK, fair enough.. But what about the fact that ##(q+p)^N=1## ? Shouldn't this mean you're taking the partial derivative of a constant, and thus it equals 0? What am I missing here?
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