`\text{~~Holi~~}`
`D=(\sqrt{15}-\sqrt{10})/(\sqrt{21}-\sqrt{14})`
`-> (\sqrt{15}-\sqrt{10})/(\sqrt{21}-\sqrt{14})×(\sqrt{21}+\sqrt{14})/(\sqrt{21}+\sqrt{14})`
`-> ((\sqrt{15}-\sqrt{10})(\sqrt{21}+\sqrt{14}))/((\sqrt{21}-\sqrt{14})(\sqrt{21}+\sqrt{14}))`
`-> (\sqrt{315}+\sqrt{210}-\sqrt{210}-\sqrt{140})/(21-14)`
`-> (3\sqrt{35}-2\sqrt{35})/7`
`-> \sqrt{35}/7`
Vậy `D=\sqrt{35}/7`