??????? Let \( f(x, y, z)=e^{x y^{2}}+4 z y^{3} \). (a) Find the directional derivative of \( f(x, y, z) \) at the point \( P_{0}(1,1,0) \) in the direction of \( \mathbf{u}=\frac{1}{3} \mathbf{i}-\frac{2}{3} 2 answers