This will be the future map . <| sep |> Instagram . More original content and fewer memes . <| sep |> The author ' s problem is that he doesn ' t understand how random variables and expected value work . He just wants to plug numbers into formulas to get answers without understanding what any of the math actually means . Take his example E x 2 , this means finding the expected value from taking a random number from the distribution of x and squaring it . The notation shows this just fine . Same with E XY , which means finding the expected value from multiplying a random value from the distribution of X with a random value from the distribution of Y . Notation works well here too . Expected value is not just some formula you plug numbers in , but a concept with a practical meaning , that it is the probability weighted mean of all outcomes , the value that a long running mean of samples approaches . If the author actually understood how expected value works , then he would not need to lookup or memorize those formulas . Take finding the formula for E x 2 for example . Each xi from x has a probability of pxi of occurring . When squaring x , you are squaring each xi , but the probabilities of each xi when squared stays the same . Squaring the number that comes from a dice roll for example does not effect the probability of which face is up after the roll . I will say that there is a disconnect between the authors that write math textbooks and students that read them . The authors seem to easily overlook pitfalls that students often find themselves in . This is because they can struggle to empathize with people who are not as familiar with math as they are .