bài 1:
a/
4x-20=0
⇔4(x-5)=0
⇔x-5=0
⇔x=5
vậy S={5}
b/
$\frac{x}{3}$ - $\frac{2x+1}{2}$ = $\frac{x}{6}$ -$x$
⇔$\frac{2x}{6}$ -$\frac{6x+3}{6}$ = $\frac{x}{6}$ - $\frac{6x}{6}$
⇔$\frac{2x-6x-3}{6}$ = $\frac{x-6x}{6}$
⇔$-4x-3$=$-5x$
⇔$-3=-5x+4x$
⇔$-3=-x$⇔$x=3$
$vậy$ $x =3$