Let U and W be subspaces of V
(a) Show that U + W is a subspace of V containing U and W.
(b) Show that span {u, w} = Ru + Rw for any vectors u and w.
(c) Show that span {u1,...um, w1...,wn} = span{u1 ..., um} + span{w1 ..., wn} for any vectors ui in U and wj in W.