+ [tex]a=-2x\Leftrightarrow \sqrt{2x+9}=-2x(-\frac{9}{2}\leq x\leq 0)\Rightarrow 2x+9=4x^2\Rightarrow 4x^2-2x-9=0\Rightarrow x=\frac{1+\sqrt{37}}{4}(loại) hoặc x=\frac{1-\sqrt{37}}{4}(t/m)[/tex]
+ [tex]a=2x+1(x\geq -\frac{1}{2})\Rightarrow \sqrt{2x+9}=2x+1\Rightarrow 2x+9=4x^2+4x+1\Leftrightarrow 4x^2+2x-8=0\Leftrightarrow 2x^2+x-4=0\Leftrightarrow x=\frac{-1+\sqrt{33}}{4}(t/m)hoặc x=\frac{-1-\sqrt{33}}{4}(loại)[/tex]