cho a,b,c dương.
ab+bc+ac\leq3abc
c/m
[tex]P=\frac{a^4b}{2a+b}+\frac{b^4c}{2b+c}+\frac{c^4a}{2c+a}\geq1[/tex]
Áp dụng BĐT Cauchy cho 5 số:
[TEX]\frac{a^4b}{2a+b}+\frac{2a+b}{9ab}+ \frac{1}{3a}+ \frac{1}{3a}+ \frac{1}{3a} \geq \frac{5}{3} \Leftrightarrow \frac{a^4b}{2a+b}+\frac{10}{9a}+\frac{2}{9b} \geq \frac{5}{3}[/TEX]
[TEX]\frac{b^4c}{2b+c}+\frac{10}{9b}+\frac{2}{9c} \geq \frac{5}{3}[/TEX]
[TEX]\frac{c^4a}{2c+a}+\frac{10}{9c}+\frac{2}{9a} \geq \frac{5}{3}[/TEX]
Cộng vế với vế lại:
[TEX]P+\frac{4}{3}(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}) \geq 5[/TEX]
[TEX]ab+bc+ac\leq3abc \Leftrightarrow \frac{1}{a}+\frac{1}{b}+\frac{1}{c} \leq 3[/TEX]
[TEX]\Rightarrow 5 \leq P+\frac{4}{3}.3 \Leftrightarrow P \geq 1[/TEX]