Tính giá trị biều thức

Ta có: !!$\left(\frac{\sqrt{14}-\sqrt{7}}{1-\sqrt{2}}+\frac{\sqrt{15}-\sqrt{5}}{1-\sqrt{3}}\right): \frac{1}{\sqrt{7}-\sqrt{5}}$ !!$=\left(\frac{\sqrt{2} \cdot \sqrt{7}-\sqrt{7}}{1-\sqrt{2}}+\frac{\sqrt{5} \cdot \sqrt{3}-\sqrt{5}}{1-\sqrt{3}}\right): \frac{1}{\sqrt{7}-\sqrt{5}}$ !!$=\left(\frac{\sqrt{7}(\sqrt{2}-1)}{1-\sqrt{2}}+\frac{\sqrt{5}(\sqrt{3}-1)}{1-\sqrt{3}}\right) \cdot(\sqrt{7}-\sqrt{5})$ !!$=(-\sqrt{7}-\sqrt{5}) \cdot(\sqrt{7}-\sqrt{5})$ !!$=-(\sqrt{7}+\sqrt{5}) \cdot(\sqrt{7}-\sqrt{5})$ !!$=-(7-5)=-2$