回復 13# HC3064 的帖子
第9題,可視為二項分布 \( \displaystyle X \sim Bin \left( 12 , \frac{1}{4} \right) \)
\( \displaystyle E(X) = np = 3 , Var(X) = np(1-p) = \frac{9}{4} , E(X^2) = Var(X) + [E(X)]^2 = \frac{45}{4} \)
所求:\( \displaystyle E(X^2 + 2X) = \frac{45}{4} + 2 \times 3 = \frac{69}{4} \)