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Em có cách nhanh hơn nè anh )))[TEX]\huge P =\frac{4}{4-x^2} + \frac{9x^2}{9x^2-1} = \frac{36x^2- 4+ 36x^2-9x^4}{-9x^4+ 37x^2-4} = \frac{-9x^4+72x^2-4}{-9x^4+37x^2-4} [/TEX]
[TEX]\huge 5P -12 = \frac{63x^4 -84x^2+28}{(4-x^2)(9x^2-1)} = \frac{7(3x-2)^2}{(4-x^2)(9x^2-1)} \ge 0 [/TEX]
[TEX]\Leftrightarrow P \ge \frac{12}{5} [/TEX]
[TEX]"=" \Leftrightarrow \left{ x^2 = \frac23 \\ y^2 = \frac32[/TEX]
[TEX]P =\frac{4}{4-x^2} + \frac{9}{9-x^2} = \frac{1}{1-\frac{x^2}{4}} + \frac{1}{1-\frac{y^2}{9}} \geq \frac{4}{2-(\frac{x^2}{4} +\frac{y^2}{9})} \geq \frac{12}{5}[/TEX]
[TEX]"=" \Leftrightarrow \left{ x^2 = \frac23 \\ y^2 = \frac32[/TEX]