The following three questions will assume the same joint distribution of X and Y as described here. Let X and Y each take on either values 1 or -1. Let p(x,y) represent P{X=x,Y=y}. Suppose E[X]=E[Y]=0. Show that p(1,1)=p(-1,-1).
Hint: if Z=X+Y, note that E[Z]=0 and compute the expectation of Z with the joint mass function of X,Y.
Show that p(-1,1)=p(1,-1).
Compute Var(X) and Var(Y).