`\qquad \sqrt{a+1}+\sqrt{a-1}<2\sqrt{a}`
`<=>\sqrt{a+1}+\sqrt{a-1}<\sqrt{a}+\sqrt{a}`
`<=>\sqrt{a+1}-\sqrt{a}<\sqrt{a}-\sqrt{a-1}` (chuyển vế)
`<=>{(\sqrt{a+1}-\sqrt{a}).(\sqrt{a+1}+\sqrt{a})}/{\sqrt{a+1}+\sqrt{a}}<{(\sqrt{a}-\sqrt{a-1}).(\sqrt{a}+\sqrt{a-1})}/{\sqrt{a}+\sqrt{a-1}}`
`<=>{a+1-a}/{\sqrt{a+1}+\sqrt{a}}<{a-(a-1)}/{\sqrt{a}+\sqrt{a-1}}`
`<=>1/{\sqrt{a+1}-\sqrt{a}}<1/{\sqrt{a}+\sqrt{a-1}}`