A = $\frac{9x}{2-x}$ + $\frac{2}{x}$ - 1 + 1 = $\frac{9x}{2-x}$ + $\frac{2-x}{x}$ + 1
Áp dụng BĐT cô si ta có:
$\frac{9x}{2-x}$ + $\frac{2-x}{x}$ $\frac{9x}{2-x}$ + $\frac{2-x}{x}$ $\geq$ 2$\sqrt[2]{\frac{9x}{2-x}.\frac{2-x}{x}}$ = 2$\sqrt[2]{9}$ = 2.3 = 6
=> A $\geq$ 6 + 1 = 7
Dấu "=" xảy ra khi:
$\frac{9x}{2-x}$ = $\frac{2-x}{x}$
=> x = 1/2 (thỏa mãn đk 0 < x < 2)
Vậy:.....