12. Вычислите: 3/4 + 4/5; 2/13 + 0; 2/3 + 1/7; 5/9 + 2/11.

1. Приводим к общему знаменателю: \(\frac34+\frac45=\frac{15}{20}+\frac{16}{20}=\frac{31}{20}=1\frac{11}{20}\).
2. \(\frac{2}{13}+0=\frac{2}{13}\).
3. \(\frac23+\frac17=\frac{14}{21}+\frac{3}{21}=\frac{17}{21}\).
4. \(\frac59+\frac{2}{11}=\frac{55}{99}+\frac{18}{99}=\frac{73}{99}\).

Ответ: \(1\frac{11}{20};\ \frac{2}{13};\ \frac{17}{21};\ \frac{73}{99}\).