From the geometry of Figure 1 we have tan ⁡ ⁡ α α = ∂ ∂ u y ∂ ∂ x d x d x + ∂ ∂ u x ∂ ∂ x d x = ∂ ∂ u y ∂ ∂ x 1 + ∂ ∂ u x ∂ ∂ x , tan ⁡ ⁡ β β = ∂ ∂ u x ∂ ∂ y d y d y + ∂ ∂ u y ∂ ∂ y d y = ∂ ∂ u x ∂ ∂ y 1 + ∂ ∂ u y ∂ ∂ y