The series are simplified for points Q and S.Taking into account that the y-coordinate does not change (dy = 0) when advancing from P to Q and neglecting higher order terms, the displacements of Q in x- and iny-direction are given by u + ∂u/∂xdx and v + ∂v/∂xdx, respectively (Fig. 3.2). Similarly, because of dx = 0, the displacement components for point S are u + ∂u/∂y dy and v + ∂v/∂ydy. During the deformation, the line PQ is transformed into line PQ. Since we assume small deformations (β 1), the lengths of PQ and of its projection onto the x-axis are approximately the same