Let a sin x 0, x = fa,b(ac) = b cos x otherwise. Let A be the set of all tuples (a, b) for which fa,b is a PDF of a continuous random variable. (a) Give an explicit description of A as a subset of R2. (b) Given (a, b) E A, find E(X) and E(sin X), if X is a continuous random variable whose PDF is fa.b