Do you think that lago proved that Desdemona and Cassio are secretly betraying Othello? Why or why not?

Because of dramatic irony, the audience knows that Desdemona and Cassio are not secretly betraying Othello. The audience knows that the entire plot of Cassio and Desdemona is concocted so that Iago can obtain the position that Othello plans not to give Iago. Iago tries to make Othello jealous so that Cassio will lose his favored position with Othello.