[PURE MATH]

3. $x^2+3x-\sqrt{2x^2+6x+1}=1$
วิธีทำ : ให้ $m=\sqrt{2x^2+6x+1}$
$\frac{m^2}{2}-\frac{1}{2} -m-1=0 $
$m^2-2m-3=0$
$(m-3)(m+1)=0$
$\therefore m=-1,3$