Đáp án:
`D=\sqrt{(2)/(2+\sqrt{3})}-\sqrt{(2)/(2-\sqrt{3})}`
`=\sqrt{(2.(2-\sqrt{3}))/((2+\sqrt{3}).(2-\sqrt{3}))}-\sqrt{(2.(2+\sqrt{3}))/((2-\sqrt{3}).(2+\sqrt{3}))}`
`=\sqrt{(4-2\sqrt{3})/(4-3)}-\sqrt{(4+2\sqrt{3})/(4-3)}`
`=\sqrt{3-2\sqrt{3}+1}-\sqrt{3+2\sqrt{3}+1}`
`=\sqrt{(\sqrt{3}-1)^2}-\sqrt{(\sqrt{3}+1)^2}`
`=|\sqrt{3}-1|-|\sqrt{3}+1|`
`=\sqrt{3}-1-(\sqrt{3}+1)(vì:\sqrt{3}>1)`
`=\sqrt{3}-1-\sqrt{3}-1`
`=-2`
