[tex]\frac{1}{R^2}=\frac{1}{OH^2}=\frac{1}{OA^2}+\frac{1}{OB^2}+\frac{1}{OC^2}=\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}=(\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2})(2^2+1^2+2^2).\frac{1}{9}\geq \frac{1}{9}.(\frac{2}{a}+\frac{1}{b}+\frac{2}{c})^2=\frac{1}{9}=>R\leq 9[/tex]
chỗ kia là bđt bunhia nha