`\qquad \sqrt{13}-\sqrt{12}`
`=(13-12)/(\sqrt{13}+\sqrt{12})`
`=1/(\sqrt{13}+\sqrt{12})`
---------------------------
`\qquad \sqrt{12}-\sqrt{11}`
`=(12-11)/(\sqrt{12}+\sqrt{11})`
`=1/(\sqrt{12}+\sqrt{11})`
Do `\sqrt{13}+\sqrt{12}>\sqrt{12}+\sqrt{11}`
`=> 1/(\sqrt{13}+\sqrt{12})<1/(\sqrt{12}+\sqrt{11})`
`=> \sqrt{13}-\sqrt{12}<\sqrt{12}-\sqrt{11}`
Vậy `\sqrt{13}-\sqrt{12}<\sqrt{12}-\sqrt{11}`
