z = e^(-xy) + yx^3 - y^2∂z/∂x = -ye^(-xy) + 3yx^2 , ∂z/∂y = -xe^(-xy) + x^3 - 2y,在 点 (1, -1), ∂z/∂x = e - 3, ∂z/∂y = -e + 3,
