1) Cho a,b,c,d > 0. Cmr:
a) $\frac{a}{b}+\frac{b}{c}+\frac{c}{a}\geq \frac{a+b+c}{^{\sqrt[3]{abc}}}$
b) $\frac{a^{2}}{b^{2}}+\frac{b^{2}}{c^{2}}+\frac{c^{2}}{d^{2}}+\frac{d^{2}}{a^{2}}\geq \frac{a+b+c+d}{\sqrt{\sqrt{abcd}}}$
2) Cho a,b,c > 0 tm $a+2b+3c$ $\geq$ 20. Tìm GTNN của:
S = $a+b+c+$$\frac{3}{a}+\frac{9}{2b}+\frac{4}{c}$