設X為空盒子數，則 P[X=3] = 0 P[X=2] = 3 * (1/3)^4 P[X=1] = 3 * [(2/3)^4 - 2*(1/3)^4] E[X] = Σ k*P[X=k] , k=1…3 上述是我看到別人的解法 請問P[X=1] = 3 * [(2/3)^4 - 2*(1/3)^4] 這一段- 2*(1/3)^4]是什麼意思