For two random variables XXX and YYY, the identity Var(X+Y)=Var(X)+Var(Y)+2Cov(X,Y)Var(X+Y) = Var(X) + Var(Y) + 2Cov(X, Y)Var(X+Y)=Var(X)+Var(Y)+2Cov(X,Y) is:
Always true for any two random variables.
Only true if XXX and YYY are independent.
Only true if XXX and YYY are Normal.
False; the correct sign is negative.