(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

From Landau and Lifgarbagez:

[tex]\langle (\Delta f)^{2} \rangle = \overline{f^{2}} - (\overline{f})^{2}[/tex]

This isn't derived, just stated, and I'd like to understand how it comes about. f is a generic quantity "relating to a macroscopic body or to a part of it."

2. Relevant equations

[tex]\Delta f = f - \overline{f}[/tex]

3. The attempt at a solution

[tex](\Delta f)^{2} = (f-\overline{f})^{2} = f^{2} - 2f \overline{f} + \overline{f}^{2}[/tex]

Basically, I don't know how to do the averaging (not without explicit values of f, anyhow).

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# Homework Help: Statistical Mechanics Derivation

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