Let (P_3) be the real vector space consisting of polynomials of degree at most 3. An element of (P_3) can be written as (p(t) = a_0 + a_1t + a_2t^2 + a_3t^3), with (a_0, a_1, a_2, a_3 in mathbb{R}).
(a) Show that (U := {p(t) in P_3 mid a_0 + a_2 = 0}) is a subspace of (P_3).
(b) Show that the vectors (1 + t, t + t^2, t^2 + t^3, t^3) are linearly independent in (P_3).