.........................................

P=.....

⇒$\frac{4}{-P}$ = $\frac{4x+8}{2\sqrt{a}-1}$ 

=$\frac{(4x-1)+9}{2\sqrt{a}-1}$ 

=$\frac{(2\sqrt{x}-1)(2\sqrt{x}+1)+9}{2\sqrt{x}-1}$ 

=2$\sqrt{x}$+1 + $\frac{9}{2\sqrt{x}-1}$ 

=(2$\sqrt{x}$-1 + $\frac{9}{2\sqrt{x}-1}$)  +2

Áp dụng BĐT cô si, ta đc:

(2$\sqrt{x}$-1 + $\frac{9}{2\sqrt{x}-1}$)  +2 $\geq$ 2$\sqrt{(2\sqrt{x}-1)\frac{9}{2\sqrt{x}-1}}$ + 2

⇔$\frac{4}{-P}$ $\geq$ 8

⇔P $\geq$ -$\frac{1}{2}$ 

Dấu bằng xảy ra ⇔ 2$\sqrt{x}$-1 = $\frac{9}{2\sqrt{x}-1}$

⇔x = 4 (TM)