Decide if each is a subspace of the vector space of real-valued functions of one real variable.
(a) The even functions {f : R → R | f(-x) = f(x) for all x}. For example, two members of this set are f1(x) = x2 and f2(x) = cos(x).
(b) The odd functions {f : R → R | f(-x) = -f(x) for all x}. Two members are f3(x) = x3 and f4(x) = sin(x).