$$\bex \sqrt{x^2+x+1}+ \sqrt{y^2+y+1} +\sqrt{x^2-x+1}+ \sqrt{y^2-y+1}\geq 2(x+y). \eex$$ Ref. [Proof Without Words: An Algebraic Inequality, The College Mathematics Journal].
[再寄小读者之数学篇](2014-11-19 一个代数不等式)
原文：http://www.cnblogs.com/zhangzujin/p/4107692.html