Đáp án:

3) $S=\left\{-\dfrac{16}{5}\right\}$

4) $S=\left\{\dfrac{5}{7}\right\}$

5) $S=\left\{\dfrac{38}{39}\right\}$

6) $S=\left\{\dfrac{1}{14}\right\}$

7) $S=\left\{\dfrac{1}{14}\right\}$

8) $S=\left\{\dfrac{25}{148}\right\}$

Giải thích các bước giải:

3)

$2(4-x)+3(2x+5)=7-x\\⇔8-2x+6x+15-7+x=0\\⇔5x+16=0\\⇔x=-\dfrac{16}{5}$

Vậy $S=\left\{-\dfrac{16}{5}\right\}$

4)

$(x-1)(x+1)-2(x-3)-x(x+5)=0\\⇔x^2-1-2x+6-x^2-5x=0\\⇔-7x+5=0\\⇔x=\dfrac{5}{7}$

Vậy $S=\left\{\dfrac{5}{7}\right\}$

5)

$8-2(4-3x)+5(7-8x)=6x-(x+3)\\⇔8-8+6x+35-40x-6x+x+3=0\\⇔-39x+38=0\\⇔x=\dfrac{38}{39}$

Vậy $S=\left\{\dfrac{38}{39}\right\}$

6)

$\dfrac{2x+8}{3}-\dfrac{4x+1}{2}=x+2\\⇔\dfrac{2x+8}{3}-\dfrac{4x+1}{2}-(x+2)=0\\⇔\dfrac{2(2x+8)-3(4x+1)-6(x+2)}{6}=0\\⇔4x+16-12x-3-6x-12=0\\⇔-14x+1=0\\⇔x=\dfrac{1}{14}$

Vậy $S=\left\{\dfrac{1}{14}\right\}$

7)

$5x-\dfrac{1-4x}{3}+\dfrac{2x-1}{4}=\dfrac{x-2}{6}+1\\⇔5x-\dfrac{1-4x}{3}+\dfrac{2x-1}{4}-\dfrac{x-2}{6}-1=0\\⇔\dfrac{12.5x-4(1-4x)+3(2x-1)-2(x-2)-12}{12}=0\\⇔60x-4+16x+6x-3-2x+4-12=0\\⇔80x-15=0\\⇔x=\dfrac{3}{16}$

Vậy $S=\left\{\dfrac{3}{16}\right\}$

8)

$\dfrac{7-3x}{5}+\dfrac{2x-1}{4}-8x=\dfrac{4+3x}{10}-\dfrac{2x+1}{2}\\⇔\dfrac{7-3x}{5}+\dfrac{2x-1}{4}-8x-\dfrac{4+3x}{10}+\dfrac{2x+1}{2}=0\\⇔\dfrac{4(7-3x)+5(2x-1)-20.8x-2(4+3x)+10(2x+1)}{20}=0\\⇔28-12x+10x-5-160x-8-6x+20x+10=0\\⇔-148x+25=0\\⇔x=\dfrac{25}{148}$

Vậy $S=\left\{\dfrac{25}{148}\right\}$