Răspuns:
1.
[tex] \frac{5a}{7b} = \frac{1}{21} = > 7b = 105a \: (produsul \: mezilor \: = \: produsul \: extremilor) \\ \frac{b}{a} = \frac{105}{7} = 15[/tex]
2.
[tex] \frac{a}{3} = \frac{5}{12} = > a = \frac{15}{12} = > 60a = 60 \times \frac{15}{12} = 75[/tex]
3.
[tex] \frac{a}{b} = \frac{x}{y} = > ay = bx = > \frac{ay}{bx} = 1 = > 50 - 1 = 49[/tex]
4.
[tex] \frac{x + 3}{12} = \frac{2}{3} = > 3(x + 3) = 24 = > x + 3 = 8 = > x = 5[/tex]
