Resposta:

[tex]a)\\ \dfrac{x}{8}=\dfrac{3}{4}\\ \\ multiplica~~cruzado\\ \\ 4x=24\\ \\ x=24\div4\\ \\\boxed{ x=6}[/tex]

[tex]b)\\ \dfrac{2x-2}{3x+1}=\dfrac{4}{7}\\ \\ 7.(2x-2)=4.(3x+1)\\ \\ 14x-14=12x+4\\ \\ 14x-12x=4+14\\ \\ 2x=18\\ \\ x=18\div2\\ \\\boxed{ x=9}[/tex]

[tex]c)\\ \dfrac{x}{x+2}=\dfrac{9}{12}\\ \\ 12x=9(x+2)\\ \\ 12x=9x+18\\ \\ 12x-9x=18\\ \\ 3x=18\\ \\ x=18\div3\\ \\\boxed{ x=6}[/tex]

[tex]d)\\ \dfrac{x}{6}=\dfrac{1,8}{4}\\ \\ 4x=6.(1,8)\\ \\ 4x=10,8\\ \\ x=10,8\div4\\ \\\boxed{ x=2,7}[/tex]

Vamos là.

do teorema de Tales vem:

a)

x/3 = 8/4

4x = 24

x = 24/4 = 6

b)

(2x - 2)/4 = (3x + 1)/7

14x - 14 = 12x + 4

14x - 12x = 4 + 14

2x = 18

x = 18/2 = 9

c)

x/9 = (x + 2)/12

12x = 9x + 18

12x - 9x = 18

3x = 18

x = 18/3 = 6

d)

x/6 = 1.8/4

4x = 6*1.8

x = 10,8/4 = 2,7