In the second case , you would need the probability function pffx instead , otherwise it is incorrect . A clear example of when this is false is to consider Uniform X from 1 to 6 a die roll and to use the function fx x 100 . The expectation of the random variable Y fX is 103 . 5 . However , the probability function in X is 0 % for all values greater than 6 . Carelessly using pxfx in Sumfxpfx would equal 0 . <| sep |> Thanks man , that was what I suspected . <| sep |> It ' s completely irrational to think Trump will get more than one supreme Court appointment <| sep |> relevant username . probably <| sep |> I ' ll do it . Bet on Rob , guys . Trust me . <| sep |> 50 % play is very good . i see him in almost every game still . <| sep |> Ok , keep telling that to yourself .