subspace and direct sum

1. Find all complex numbers z which satisfy z^2 = -4i.

2. Prove that {p(x) <- P (F) I p'(x) = 0} is a subspace of P (F).

3. Let V = R^3 and let U = { x y z} <- R^3 I x+y=1}.
(a) Find a subspace W1 of R^3 such that V is not equal to U + W1..
(b) Find a subspace W2 of R^3 such that V = U + W2 but V is the equal to the
direct sum of U and W2 .

Thx.