Rút gọn biểu thức:
$$P=\left(\frac{\sqrt[3]{x}-\sqrt[3]{y}}{\sqrt[3]{x^{2}y}+\sqrt[3]{x{y}^2}}+\frac{\sqrt[3]{x}+\sqrt[3]{y}}{\sqrt[3]{x^{2}y}-\sqrt[3]{x{y}^2}}\right)\frac{x\sqrt[3]{y}}{\sqrt[3]{x^2}+\sqrt[3]{y^2}}-\frac{2\sqrt[3]{y^2}}{\sqrt[3]{x^2}-\sqrt[3]{y^2}}(\text{với }x>0,y>0,x\ne y).$$