7. The value of ∫_0^1 x√1 - x dx is (7).

4. Let \( u = x + y + z \), \( v = xy + yz + zx \), \( w = xyz \), and suppose that \( f(u, v, w) \) is a differentiable function satisfying \( f(u, v, w) = x^4 + y^4 + z^4 \) for all \( (x, y, z) \). Then, \( f_u(2, -1, -2) = \) (4).