Theo BĐT Cauchy-Schwarz ta có:Cho $x> 0;y> 0;x+y<1$. Chứng minh $\frac{1}{x^2+xy}+\frac{1}{y^2+xy}\geq 4$
$\frac{1}{{{x^2} + xy}} + \frac{1}{{{y^2} + xy}} \ge \frac{4}{{{x^2} + {y^2} + 2xy}} = \frac{4}{{{{\left( {x + y} \right)}^2}}} > 4\,\,\left( {do\,x + y < 1} \right)$