X is a random variable with E(x) = 1.5 and σ = 0.8 E(7-4X+2X^2) =

题目

X is a random variable with E(x) = 1.5 and σ = 0.8 E(7-4X+2X^2) =
Complete the sentence by typing your answers into the textboxes
X is a random variable with E(x) = 1.5 and σ = 0.8
E(7-4X+2X^2) = (specify with 2 digits)
回答的第一条 错误
算出来本应是6.78 但题目要求是 两位数 四舍五入后6.8 错误

答案

由 σ^2=Ex^2-(Ex)^2得Ex^2=σ^2+(Ex)^2=2.89
故E(7-4X+2X^2) =7-4*Ex+2*Ex^2=7-6+2*2.89=6.78约等于6.8