[กิตติ]

4) $x^2-\frac{a+b}{ab}x+\frac{1}{ab}=0$

$abx^2-(a+b)x+1=0$
$(ax-1)(bx-1)=0$
$x=\frac{1}{a}, \frac{1}{b} $

5) $x+\frac{1}{x}=2$

$x^2-2x+1=2$
$(x-1)^2=0$
$x=1$

14) $\sqrt{\frac{a+x}{b+x}}-\sqrt{\frac{b+x}{a+x}}=\frac{3}{2}$

$\sqrt{\frac{a+x}{b+x}} = A$
$\sqrt{\frac{b+x}{a+x}} = \frac{1}{A} $
$A-\frac{1}{A} = \frac{3}{2}$
$2A^2-3A-2 =0$
$(2A+1)(A-2)=0$
$A=2$
$\sqrt{\frac{a+x}{b+x}} = 2$
$\frac{a+x}{b+x} = 4$
$a+x = 4b+4x$
$x= \frac{a-4b}{3} $