In the video it says that the formula of the variance has been presented before, but I can't find references. I don't understand what to replace in E(y squared), while it is clear that E(y) squared is (np)squared

Hey Davide, The trouble is that, unfortunately, the derivation is not as trivial. There is a nice proofwiki page on the topic. This page shows the derivation of E(X) for a binomial distribution, namely E(X) = np I think that Proof 1 would be easier to follow as it uses the formula for P(X) introduced in the video. This page shows the derivation of E(X^2) which is even less pleasant to look at :) At the end of the day, one obtains E(X^2) = n^2*p^2 + np(1-p) Again, Proof 1 should be easier to follow. It is now straightforward to obtain the result in the video lecture: sigma^2 = E(X^2) - E(X)^2 = np(1-p) Hope that you will find the resources helpful! Kind regards, 365 Hristina