The triangular stress element must satisfy equilibrium; therefore, we must convert the stresses on each face to their corresponding forces. To do this, we must define the area of each face.

Let the area of the n face be denoted as Δ A.

The area of the vertical face (i.e., the –x face) will be Δ A  cos θ.

The area of the horizontal face (i.e., the –y face) will be Δ A  sin θ.