The second element requiring explanation is the approximation of the values of stress acting on the walls opposite the walls covering the axes. Let us focus on the right wall where the stress is an approximation of stress σ σ x x from the left wall at points with coordinates x + d x and it is equal to σ σ x x + ∂ ∂ σ σ x x ∂ ∂ x d x . This approximation suffices since, as d x goes to zero, σ σ x x ( x + d x ) − − ( σ σ x x ( x ) + d x ∂ ∂ σ σ x x ( x ) ∂ ∂ x ) d x = σ σ x x ( x + d x ) − − σ σ x x ( x ) d x − − ∂ ∂ σ σ x x ( x ) ∂ ∂ x goes to zero as well, by definition of partial derivative.