669 а) \(\frac{8}{10}\), \(\frac{4}{6}\), \(\frac{6}{9}\), \(\frac{10}{25}\), \(\frac{14}{49}\), \(\frac{15}{20}\), \(\frac{12}{15}\), \(\frac{14}{18}\); б) \(\frac{8}{12}\), \(\frac{12}{16}\), \(\frac{12}{18}\), \(\frac{24}{40}\), \(\frac{18}{27}\), \(\frac{20}{70}\), \(\frac{8}{36}\), \(\frac{30}{75}\).

Сократим дроби, разделив числитель и знаменатель на их общий делитель:

1. а)

   • \(\frac{8}{10} = \frac{2 \cdot 4}{2 \cdot 5} = \frac{4}{5}\)
   • \(\frac{4}{6} = \frac{2 \cdot 2}{2 \cdot 3} = \frac{2}{3}\)
   • \(\frac{6}{9} = \frac{3 \cdot 2}{3 \cdot 3} = \frac{2}{3}\)
   • \(\frac{10}{25} = \frac{5 \cdot 2}{5 \cdot 5} = \frac{2}{5}\)
   • \(\frac{14}{49} = \frac{7 \cdot 2}{7 \cdot 7} = \(\frac{2}{7}\)
   • \(\frac{15}{20} = \frac{5 \cdot 3}{5 \cdot 4} = \frac{3}{4}\)
   • \(\frac{12}{15} = \frac{3 \cdot 4}{3 \cdot 5} = \frac{4}{5}\)
   • \(\frac{14}{18} = \frac{2 \cdot 7}{2 \cdot 9} = \frac{7}{9}\)

2. б)

   • \(\frac{8}{12} = \frac{4 \cdot 2}{4 \cdot 3} = \frac{2}{3}\)
   • \(\frac{12}{16} = \frac{4 \cdot 3}{4 \cdot 4} = \frac{3}{4}\)
   • \(\frac{12}{18} = \frac{6 \cdot 2}{6 \cdot 3} = \frac{2}{3}\)
   • \(\frac{24}{40} = \frac{8 \cdot 3}{8 \cdot 5} = \frac{3}{5}\)
   • \(\frac{18}{27} = \frac{9 \cdot 2}{9 \cdot 3} = \frac{2}{3}\)
   • \(\frac{20}{70} = \frac{10 \cdot 2}{10 \cdot 7} = \frac{2}{7}\)
   • \(\frac{8}{36} = \frac{4 \cdot 2}{4 \cdot 9} = \frac{2}{9}\)
   • \(\frac{30}{75} = \frac{15 \cdot 2}{15 \cdot 5} = \frac{2}{5}\)

Ответ: сокращенные дроби указаны в решении.