`@Mon`
`\text{ Ta có:}`
`(\frac{\sqrt{14}-\sqrt{7}}{1-\sqrt{2}}+\frac{\sqrt{15}-\sqrt{5}}{1-\sqrt{3}}):\frac{1}{\sqrt{7}-\sqrt{5}}`
`=[\frac{\sqrt{7}(\sqrt{2}-1)}{1-\sqrt{2}}+\frac{\sqrt{5}(\sqrt{3}-1)}{1-\sqrt{3}}].(\sqrt{7}-\sqrt{5})`
`=(-\sqrt{7}-\sqrt{5})(\sqrt{7}-\sqrt{5})`
`=-(\sqrt{7}+\sqrt{5})(\sqrt{7}-\sqrt{5})`
`=-(7-5)`
`=-2(đpcm)`
`Vậy` `(\frac{\sqrt{14}-\sqrt{7}}{1-\sqrt{2}}+\frac{\sqrt{15}-\sqrt{5}}{1-\sqrt{3}}):\frac{1}{\sqrt{7}-\sqrt{5}}=-2`