Cho ba số $a,b,c$ không âm, tìm GTNN của
$P=\frac{\sqrt{ab}}{c+3\sqrt{ab}}+\frac{\sqrt{bc}}{a+3\sqrt{bc}}+\frac{\sqrt{ca}}{b+3\sqrt{ca}}$
Theo Bunhiacopxki và Cosi ta có :
$\sum \frac{c}{c+3\sqrt{ab}}\geq \sum \frac{c}{c+3.\frac{a+b}{2}}=2\sum \frac{c}{2c+3a+3b}=2\sum \frac{c^2}{2c^2+3ac+3bc}\geq 2.\frac{(\sum c)^2}{2\sum c^2+6\sum ab}=\frac{2(\sum c)^2}{2(\sum c)^2+2\sum ab}\geq \frac{2(\sum c)^2}{2(\sum c)^2+\frac{2(\sum c)^2}{3}}=\frac{6(\sum c)^2}{8(\sum c)^2}=\frac{6}{8}=\frac{3}{4}= > \sum \frac{c}{c+3\sqrt{ab}}\geq \frac{3}{4}= > \sum (1-\frac{c}{c+3\sqrt{ab}})\leq \frac{9}{4}= > P=\sum \frac{\sqrt{ab}}{c+3\sqrt{ab}}\leq \frac{3}{4}$