Show that if w and Z are subspaces of V. then
(a) Their intersection W ∩ Z is a subspace of V,
(b) Their sum W + Z = { w + z | w ∈ W. z ∈ Z ) is also a subspace, but
(c) Their union WUZ is not a subspace of V, unless W ⊂ Z or Z ⊂ W.