No.003 A

期待値は以下の計算式になる.

$$\begin{align} E &= \frac{1}{N} + \frac{2}{N} + \dots + \frac{N}{N} \\ &= \frac{\sum_{i=1}^N i}{N} \\ &= \frac{N(N+1)}{2} \frac{1}{N} \\ &= \frac{N+1}{2} \end{align}$$