Resposta:
Basta multiplicar cruzado. Espero que tenha entendido :)
Explicação passo-a-passo:
a)
[tex] \frac{x - 2}{12} = \frac{x}{20} \\ \\ 20 \times (x - 2) = 12x \\ 20x - 40 = 12x \\ 20x - 12x = 40 \\ 8x = 40 \\ x = \frac{40}{8} \\ \\ x = 5[/tex]
b)
[tex] \frac{x + 1}{5} = \frac{x}{3} \\ \\ 3 \times (x + 1) = 5x \\ 3x + 3 = 5x \\ 3x - 5x = - 3 \\ - 2x = - 3 \\ x = \frac{ - 3}{ - 2} \\ \\ x = \frac{3}{2} \: ou \: 1.5[/tex]
c)
[tex] \frac{x - 1}{7} = \frac{16}{28} \\ \\ 28 \times (x - 1) = 7 \times 16 \\ 28x - 28 = 112 \\ 28x = 112 + 28 \\ 28x = 140 \\ x = \frac{140}{28} \\ \\ x = 5[/tex]
d)
[tex] \frac{x + 3}{x - 2} = \frac{3}{2} \\ \\ 2 \times (x + 3) = 3 \times (x - 2) \\ 2x + 6 = 3x - 6 \\ 2x - 3x = - 6 - 6 \\ - x = - 12 \\ x = 12[/tex]
e)
[tex] \frac{20}{x + 1} = \frac{16}{x} \\ \\ 20x = 16 \times (x + 1) \\ 20 x = 16x + 16 \\ 20x - 16x = 16 \\ 4x = 16 \\ x = \frac{16}{4} \\ \\ x = 4[/tex]
f)
[tex] \frac{3}{2} = \frac{x + 3}{x - 1} \\ \\ 3 \times (x - 1) = 2 \times (x + 3) \\ 3x - 3 = 2x + 6 \\ 3x - 2x = 6 + 3 \\ x = 9[/tex]
