Question

弱问~DX=E[(E-EX)^2]为什么能推出DX=E(X^2)-(EX)^2?

Answer 1

DX=E[(X-EX)^2]=E[X^2-2XE(X)+(EX)^2]=E[X^2]-2E(X)E(X)+(EX)^2=E[X^2]-(EX)^2概率统计要都死记,才会感觉是门很难的课...vice versa