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 $x=-\dfrac{5}{6}$

Cách giải:

`18,(2(1-3x))/5-(2+3x)/10=7-(3(2x+1))/4`

`->(2-6x)/5-(2+3x)/10=7-(6x+3)/4`

`->(4-12x-2-3x)/10=(28-6x-3)/4`

`->(2-15x)/10=(25-6x)/4`

`->8-60x=250-60x`

`->0=242` vô lý

`->` pt vô nghiệm.

`19,(5x+2)/6-(8x-1)/6=(4x+2)/5-5`

`->(5x+2-16x+2)/6=(4x+2-25)/5`

`->(-9x+4)/6=(4x-23)/5`

`->24x-138=-45x+20`

`->69x=138`

`->x=138/69=2`

Vậy `x=1`

`20,(3x+2)/2-(3x+1)/6=2x+5/3`

`->(9x+6-3x+1)/6=(6x+10)/6`

`->6x+6x+10=10`

`->12x=0`

`->x=0`

Vậy `x=0`