x=1时概率为p,那么x的期望为p ==>
P(x=1) = p; P(x=0) = 1-p ==>
P(x^3=1) = p; P(x^3 = 0) = 1-p ==>
x的三次方的期望 = 1 * p + 0 * (1-p) = p