Hello,
use this formula of the covariance :
cov(X,Y)=E[XY]-E[X]E[Y]
as for computing E[XY], think that XY=1 or 0, and find its probability in each case.
Question : X and Y are indicator variables for whether or not the events A and B occur, i.e.
Compute cov(X,Y) If cov(X,Y)>0, show that P(Y=1|X=1)> P(Y=1)
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......for cov(X,Y)>0 and others
Hey, it's E[XY] (a product), not E[X,Y]
Or you could've seen it this way : XY = 0 or 1, according to the values of X and Y. So it follows a Bernoulli distribution, with parameter P(XY=1)=P(X=1,Y=1)
Plus, X is not an event, so we don't talk about "X occurring". Rather "A occurring". And nothing tells you that the probability is 1/2 !!! You have to keep them in formal form.