Show that the compatibility equation for the case of plane strain, namely, \frac{\partial^{2} \gamma_{x y}}{\partial x \partial y}=\frac{\partial^{2} \varepsilon_{y}}{\partial x^{2}}+\frac{\partial^{2} \varepsilon_{x}}{\partial y^{2}} in the formOymay be expressed in terms of

direct stresses oy and \left(\frac{\partial^{2}}{\partial x^{2}}+\frac{\partial^{2}}{\partial y^{2}}\right)\left(\sigma_{x}+\sigma_{y}\right)=0