Giải thích các bước giải:
18) $\dfrac{2(1-3x)}{5}-\dfrac{2+3x}{10}=7-\dfrac{3(2x+1}{4}$
$⇒\dfrac{2.4(1-3x)}{20}-\dfrac{2(2+3x)}{20}=\dfrac{140}{20}-\dfrac{3.5(2x+1)}{20}$
$⇒8(1-3x)-2(2+3x)=140-15(2x+1)$
$⇒8-24x-4-6x=140-30x-15$
$⇒4-30x=-30x-125$
$⇒129=0_\text{(vô lý)}$
Vậy phương trình vô nghiệm
19) $\dfrac{5x+2}{6}-\dfrac{8x-1}{3}=\dfrac{4x+2}{5}-5$
$⇒\dfrac{5(5x+2)}{30}-\dfrac{10(8x-1)}{30}=\dfrac{6(4x+2)}{30}-\dfrac{150}{30}$
$⇒5(5x+2)-10(8x-1)=6(4x+2)-150$
$⇒25x+10-80x+10=24x+12-150$
$⇒-55x+20=24x-138$
$⇒79x=-158$
$⇒x=-2$
Vậy $x=-2$
20) $\dfrac{3x+2}{2}-\dfrac{3x+1}{6}=2x+\dfrac{5}{3}$
$⇒\dfrac{3(3x+2)}{6}-\dfrac{3x+1}{6}=\dfrac{12x}{6}+\dfrac{10}{6}$
$⇒3(3x+2)-3x-1=12x+10$
$⇒9x+6-3x-1=12x+10$
$⇒6x+5=12x+10$
$⇒-6x=5$
$⇒x=-\dfrac{5}{6}$
Vậy $x=-\dfrac{5}{6}$
