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A quick review of resolving expectations: If you know that a discrete random variable $X$, drawn from set $\mathcal{X}$ has probability distribution $p(x) = \mathbf{Pr}\{X=x \}$, then $$\mathbb{E}[X] = \sum_{x \in \mathcal{X}} xp(x)$$ This equation is the core of what is going on when resolving the expectation in your quoted equation. Resolving the ...