[tex]GT\Rightarrow 0 \leq a;b;c \leq 1\\\Leftrightarrow \left\{\begin{matrix} & a(a-1)\leq 0 & \\ & b(b-1)\leq 0 & \\ & c(c-1)\leq 0 & \\ \end{matrix}\right.\\\Leftrightarrow \left\{\begin{matrix} & a\geq a^2 & \\ & b\geq b^2 & \\ & c\geq c^2 & \end{matrix}\right.[/tex]
Có: [tex]\displaystyle \sum _{cyc} \sqrt{7a+9}\geq \displaystyle \sum _{cyc} \sqrt{a^2+6a+9}= \sum _{cyc} a+9=1+9=10[/tex]