Let X and Y be two random variables and let r, s, t, and u be arbitrary real numbers
a. Derive from the definition that Cov(X + s, Y + u) = Cov(X,Y).
b. Derive from the definition that Cov(rX.tY) = rtCov(X,Y).
c. Con nbine parts a and b to show Cov(rX + s,tY + u) = rtCov(X,Y).