Expected value of U.

Let Y1, Y2, . . . , Yn be independent, uniformly distributed random variables on the interval [0, θ]. a) Find the joint density function of Y( j ) and Y(k) where j and k are integers 1 ≤ j < k ≤ n.

b) Use the result from part (a) to find Cov(Y( j ), Y(k)) when j and k are integers 1 ≤ j < k ≤ n.

Cov(Y( j ), Y(k))=E(Y(j)*Y(k))-E(Y(j))*E(Y(k)) But I can’t find the value of E(Y(j)*Y(k)). Help me ;-(