[Toán 9] Tìm GTNN

T

thanhlan9

ĐK: [TEX]1-x^2 > 0[/TEX]
Ta có [TEX]A^2=\frac{9x^2-30x+25}{1-x^2}=16+ \frac{25x^2-30x+9}{1-x^2}=16+ \frac{(5x-3)^2}{1-x^2} \geq 16 \Rightarrow A \geq 4[/TEX]
Dấu = xảy ra khi x=3/5 (thỏa mãn)
 
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